Image.XWD: Further improve size check

Thanks to Cezary Cerekwicki <ccerekwicki@opera.com> for the report.

src/modules/Image/encodings/xwd.c

@@ -302,7 +302,7 @@ void img_xwd__decode(INT32 args,int header_only,int skipcmap)
       n++;
       push_text("image");
 
-      if (s->len-(unsigned int)(header.header_size+header.ncolors*12)<0)
+      if ((size_t)s->len < (header.header_size+header.ncolors*12))
 	 push_empty_string();
       else
 	 push_string(make_shared_binary_string(