tdf#120703 PVS: V1028 fix unexpected integer overflow

V1028 Possible overflow. Consider casting arguments of the operator
      to the type instead of casting its result.

V547 Expression 'nNewSize > ((sal_Int32) 0x7FFFFFFF)' is always false.

Change-Id: I7107b162d88c9d3f97019effcac1f9cfab52a66c
Reviewed-on: https://gerrit.libreoffice.org/62846Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>

comphelper/source/streaming/memorystream.cxx

@@ -190,7 +190,7 @@ void SAL_CALL UNOMemoryStream::writeBytes( const Sequence< sal_Int8 >& aData )
     const sal_Int32 nBytesToWrite( aData.getLength() );
     if( nBytesToWrite )
     {
-        sal_Int64 nNewSize = static_cast< sal_Int64 >( mnCursor + nBytesToWrite );
+        sal_Int64 nNewSize = static_cast<sal_Int64>(mnCursor) + nBytesToWrite;
         if( nNewSize > SAL_MAX_INT32 )
         {
             OSL_ASSERT(false);