tdf#93949 ensure memory stream is properly flushed

Change-Id: I6d6a926f5d3fd62dd3b7b78a5721f6483b3b4ee7
(cherry picked from commit c6b11cf6)
Reviewed-on: https://gerrit.libreoffice.org/20842Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>
(cherry picked from commit a0a35440)

sc/source/filter/excel/xechart.cxx

@@ -720,6 +720,7 @@ void XclExpChEscherFormat::WriteBody( XclExpStream& rStrm )
     // write Escher property container via temporary memory stream
     SvMemoryStream aMemStrm;
     maData.mxEscherSet->Commit( aMemStrm );
+    aMemStrm.Flush();
     aMemStrm.Seek( STREAM_SEEK_TO_BEGIN );
     rStrm.CopyFromStream( aMemStrm );
 }