Avoid storing potentially negative and unused value in sal_uInt16 var

At least during CppunitTest_sd_misc_tests, when bReplace is true the value
initially calculated for nSdPageEnd (and later overwritten) was -1, so caused a
warning with Clang's implicit-signed-integer-truncation.

Change-Id: I69be2649e431590fc79808908470c9c2e3cf3c74
Reviewed-on: https://gerrit.libreoffice.org/68663
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sd/source/core/drawdoc3.cxx

@@ -789,17 +789,13 @@ bool SdDrawDocument::InsertBookmarkAsPage(
     if (nInsertPos > 0)
     {
         sal_uInt16 nSdPageStart = (nInsertPos - 1) / 2;
-        sal_uInt16 nSdPageEnd = GetSdPageCount(PageKind::Standard) - nSdPageCount +
-                            nSdPageStart - 1;
+        sal_uInt16 nSdPageEnd = bReplace
+            ? nSdPageStart + nReplacedStandardPages - 1
+            : GetSdPageCount(PageKind::Standard) - nSdPageCount + nSdPageStart - 1;
         const bool bRemoveEmptyPresObj =
                 (pBookmarkDoc->GetDocumentType() == DocumentType::Impress) &&
                 (GetDocumentType() == DocumentType::Draw);
 
-        if( bReplace )
-        {
-            nSdPageEnd = nSdPageStart + nReplacedStandardPages - 1;
-        }
-
         std::vector<OUString>::iterator pExchangeIter;
 
         if (pExchangeList)