cid#1608288 silence Overflowed constant

Change-Id: Ic20c62c06f67792b263a15adcc4855297f2fefef
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/170823
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

vcl/source/filter/egif/giflzwc.cxx

@@ -53,6 +53,7 @@ inline void GIFImageDataOutputStream::FlushBitsBufsFullBytes()
         if( nBlockBufSize==255 )
             FlushBlockBuf();
 
+        assert(nBlockBufSize != 255 && "FlushBlockBuf ensures this is incrementable without overflow");
         pBlockBuf[nBlockBufSize++] = static_cast<sal_uInt8>(nBitsBuf);
         nBitsBuf >>= 8;
         nBitsBufSize -= 8;