tools: don't leave SvStream::m_nBufFilePos pointing beyond the end

... of the stream in SvStream::SetStreamSize(); this caused
SvMemoryStream with SetStreamSize(0) and subsequent write to be
pre-filled with 0 bytes.

Change-Id: I0de704b319f5087bc6c1914881e38018212afbf2
Reviewed-on: https://gerrit.libreoffice.org/80478
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@cib.de>

tools/source/stream/stream.cxx

@@ -1561,6 +1561,10 @@ bool SvStream::SetStreamSize(sal_uInt64 const nSize)
     sal_uInt16 nBuf = m_nBufSize;
     SetBufferSize( 0 );
     SetSize( nSize );
+    if (nSize < m_nBufFilePos)
+    {
+        m_nBufFilePos = nSize;
+    }
     SetBufferSize( nBuf );
 #ifdef DBG_UTIL
     DBG_ASSERT(Tell()==nFPos,"SetStreamSize failed");