sot: fix some bizarre resize code that creates 4G temp files on 32bit

StgTmpStrm::SetSize(): the nEndOfData - 1 may underflow if nEndOfData=0
which happened to be the same value of STREAM_SEEK_TO_END before
but not any more; also it's a mystery to me why the last byte in the
buffer would need to be overwritten with 0 here.

This also fixes the failure of ww8_export test.

Change-Id: Iefe5a22c667ee83fbc315d772f357b913105ad93

sot/source/sdstor/stgstrms.cxx

@@ -1241,10 +1241,7 @@ void StgTmpStrm::SetSize(sal_uInt64 n)
         {
             if( n > nEndOfData )
             {
-                sal_uInt64 nCur = Tell();
-                Seek( nEndOfData - 1 );
-                WriteUChar( (sal_uInt8) 0 );
-                Seek( nCur );
+                SvMemoryStream::SetSize(n);
             }
             else
                 nEndOfData = n;