sw: fix UBSAN error in CppunitTest_sw_odfimport

/sw/source/core/layout/calcmove.cxx:1686:43: runtime error: downcast of address 0x2ae9ba4566c0 which does not point to an object of type 'const SwFootnoteFrame'
0x2ae9ba4566c0: note: object is of type 'SwSectionFrame'

Footnotes can contain sections of course, so use FindFootnoteFrame().

(regression from c83a443e)

Change-Id: I7824c81ba74ed3341f0ba5b14dac89d71f6318db
Reviewed-on: https://gerrit.libreoffice.org/39157Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>

sw/source/core/layout/calcmove.cxx

@@ -1683,7 +1683,8 @@ void SwContentFrame::MakeAll(vcl::RenderContext* /*pRenderContext*/)
         aRectFnSet.Refresh(this);
         if (!bMovedFwd && bFootnote && GetIndPrev() != pPre)
         {   // SwFlowFrame::CutTree() could have formatted and deleted pPre
-            auto const pPrevFootnoteFrame(static_cast<SwFootnoteFrame const*>(GetUpper())->GetMaster());
+            auto const pPrevFootnoteFrame(static_cast<SwFootnoteFrame const*>(
+                        FindFootnoteFrame())->GetMaster());
             bool bReset = true;
             if (pPrevFootnoteFrame)
             {   // use GetIndNext() in case there are sections