replace an OSL_ENSURE() with assert(), tdf#89319 related

Commit 8e2293e0 changed
STREAM_SEEK_TO_END logic, ensure that in a dbgutil build we catch
unnoticed corner cases with assert() instead of just another fly-by
warning.

Change-Id: If6c4fb4d2467f7dc7b8177caf7353d0b0e56ed0d

sc/source/filter/excel/xestream.cxx

@@ -382,7 +382,7 @@ sal_uInt64 XclExpStream::SetSvStreamPos(sal_uInt64 const nPos)
 
 void XclExpStream::InitRecord( sal_uInt16 nRecId )
 {
-    OSL_ENSURE( mrStrm.remainingSize() == 0, "XclExpStream::InitRecord - not in end position" );
+    assert(mrStrm.remainingSize() == 0);    // stream must be at end position
     mrStrm.WriteUInt16( nRecId );
 
     mnLastSizePos = mrStrm.Tell();