app: don't try to add a NULL boundary to GimpBrush's boundary cache

app/core/gimpbrush.c

@@ -668,6 +668,13 @@ gimp_brush_transform_boundary (GimpBrush *brush,
                                                                    width,
                                                                    height);
 
+      /*  while the brush mask is always at least 1x1 pixels, its
+       *  outline can correctly be NULL
+       *
+       *  FIXME: make the cache handle NULL things when it is
+       *         properly implemented
+       */
+      if (boundary)
         gimp_brush_cache_add (brush->boundary_cache,
                               (gpointer) boundary,
                               *width, *height,