Avoid computing negative nColSpan

With Clang -fsanitize=implicit-signed-integer-truncation,
CppunitTest_sw_ooxmlexport8 fails with

> sw/source/filter/writer/wrtswtbl.cxx:666:39: runtime error: implicit conversion from type 'int' of value -1 (32-bit, signed) to type 'sal_uInt16' (aka 'unsigned short') changed the value to 65535 (16-bit, unsigned)
>  #0 in SwWriteTable::FillTableRowsCols(long, unsigned short, unsigned int, unsigned short, long, unsigned int, SwTableLines const&, SvxBrushItem const*, unsigned short, unsigned short) at sw/source/filter/writer/wrtswtbl.cxx:666:39
>  #1 in SwWriteTable::SwWriteTable(SwTable const*, SwTableLines const&, long, unsigned int, bool, unsigned short, unsigned short, unsigned short, unsigned int) at sw/source/filter/writer/wrtswtbl.cxx:757:5
>  #2 in DocxAttributeOutput::InitTableHelper(std::shared_ptr<ww8::WW8TableNodeInfoInner> const&) at sw/source/filter/ww8/docxattributeoutput.cxx:3481:31
>  #3 in DocxAttributeOutput::FinishTableRowCell(std::shared_ptr<ww8::WW8TableNodeInfoInner> const&, bool) at sw/source/filter/ww8/docxattributeoutput.cxx:907:9
>  #4 in DocxAttributeOutput::TableNodeInfoInner(std::shared_ptr<ww8::WW8TableNodeInfoInner>) at sw/source/filter/ww8/docxattributeoutput.cxx:4248:5
>  #5 in MSWordExportBase::OutputStartNode(SwStartNode const&) at sw/source/filter/ww8/wrtww8.cxx:4167:26
>  #6 in MSWordExportBase::WriteText() at sw/source/filter/ww8/wrtww8.cxx:2812:13
>  #7 in DocxAttributeOutput::WriteFloatingTable(ww8::Frame const*) at sw/source/filter/ww8/docxattributeoutput.cxx:336:17
>  #8 in checkAndWriteFloatingTables(DocxAttributeOutput&) at sw/source/filter/ww8/docxattributeoutput.cxx:396:30
>  #9 in DocxAttributeOutput::StartParagraph(std::shared_ptr<ww8::WW8TableNodeInfo>) at sw/source/filter/ww8/docxattributeoutput.cxx:406:9
>  #10 in MSWordExportBase::OutputTextNode(SwTextNode&) at sw/source/filter/ww8/wrtw8nds.cxx:2181:22
>  #11 in MSWordExportBase::OutputContentNode(SwContentNode&) at sw/source/filter/ww8/wrtw8nds.cxx:3327:13
>  #12 in MSWordExportBase::WriteText() at sw/source/filter/ww8/wrtww8.cxx:2727:13
>  #13 in DocxExport::WriteMainText() at sw/source/filter/ww8/docxexport.cxx:1469:5
>  #14 in DocxExport::ExportDocument_Impl() at sw/source/filter/ww8/docxexport.cxx:527:5
>  #15 in MSWordExportBase::ExportDocument(bool) at sw/source/filter/ww8/wrtww8.cxx:3212:19
>  #16 in DocxExportFilter::exportDocument() at sw/source/filter/ww8/docxexportfilter.cxx:86:17
>  #17 in oox::core::FilterBase::filter(com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&) at oox/source/core/filterbase.cxx:497:55
>  #18 in WriterFilter::filter(com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&) at writerfilter/source/filter/WriterFilter.cxx:149:23
>  #19 in SfxObjectShell::ExportTo(SfxMedium&) at sfx2/source/doc/objstor.cxx:2411:25
>  #20 in SfxObjectShell::SaveTo_Impl(SfxMedium&, SfxItemSet const*) at sfx2/source/doc/objstor.cxx:1505:19
>  #21 in SfxObjectShell::PreDoSaveAs_Impl(rtl::OUString const&, rtl::OUString const&, SfxItemSet const&) at sfx2/source/doc/objstor.cxx:2818:39
>  #22 in SfxObjectShell::CommonSaveAs_Impl(INetURLObject const&, rtl::OUString const&, SfxItemSet&) at sfx2/source/doc/objstor.cxx:2675:9
>  #23 in SfxObjectShell::APISaveAs_Impl(rtl::OUString const&, SfxItemSet&) at sfx2/source/doc/objserv.cxx:321:19
>  #24 in SfxBaseModel::impl_store(rtl::OUString const&, com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&, bool) at sfx2/source/doc/sfxbasemodel.cxx:3001:46
>  #25 in SfxBaseModel::storeToURL(rtl::OUString const&, com::sun::star::uno::Sequence<com::sun::star::beans::PropertyValue> const&) at sfx2/source/doc/sfxbasemodel.cxx:1672:13
>  #26 in SwModelTestBase::reload(char const*, char const*, char const*) at sw/qa/extras/inc/swmodeltestbase.hxx:796:20
>  #27 in SwModelTestBase::executeImportExportImportTest(char const*, char const*) at sw/qa/extras/inc/swmodeltestbase.hxx:283:9
>  #28 in testN779642::Import_Export_Import() at sw/qa/extras/ooxmlexport/ooxmlexport8.cxx:746:1

(where nCPos is 23, nOldCol is 2, and nCol is 0).

sw/qa/extras/ooxmlexport/data/n779642.docx processed by that testN779642 is the
only case during `make check` for which the added SAL_WARN_IF triggers.  That
file was added with 874f0a1b "n#779642: ignore
framePr inside table or the table import won't work", leaving its origin
unclear, and trying to open it with at least MS Word for Mac version 16.21 fails
with "could not be opened", so it looks somewhat plausible that computation of
such negative spans should just not happen.

Change-Id: I0d460d04050017f7793ae51dc2a51ebbd3c4c193
Reviewed-on: https://gerrit.libreoffice.org/67075
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

sw/source/filter/writer/wrtswtbl.cxx

@@ -663,7 +663,8 @@ void SwWriteTable::FillTableRowsCols( long nStartRPos, sal_uInt16 nStartRow,
                 else if ( nAttrRowSpan < 1 )
                     nRowSpan = 0;
 
-                sal_uInt16 nColSpan = nCol - nOldCol + 1;
+                SAL_WARN_IF(nCol < nOldCol, "sw.filter", "unexpected " << nCol << " < " << nOldCol);
+                sal_uInt16 nColSpan = nCol >= nOldCol ? nCol - nOldCol + 1 : 1;
                 pRow->AddCell( pBox, nOldRow, nOldCol,
                                nRowSpan, nColSpan, nHeight,
                                pBrushItem );