Issue #4605 - Invalid abr brush crashes GIMP

When loading ABR brushes, error out on negative scan-line lengths,
instead of trying to allocate a huge buffer.  This entire code
could use a whole lot more unsignedness, but this should cover the
crash.

app/core/gimpbrush-load.c

@@ -1131,7 +1131,7 @@ abr_rle_decode (GDataInputStream  *input,
   for (i = 0; i < height; i++)
     {
       cscanline_len[i] = abr_read_short (input, error);
-      if (error && *error)
+      if ((error && *error) || cscanline_len[i] <= 0)
         goto err;
     }