core: Fix crash when filling with Middle Gray color

Filling with Middle Gray requires getting the image format
for conversion from CIE Lab. This works when filling a layer,
but new images are not yet available in the context.
This adds the image to the context before gimp_drawable_fill ()
is called so it has access to the image format.

app/core/gimpimage-new.c

@@ -112,6 +112,7 @@ gimp_image_new_from_template (Gimp         *gimp,
                              gimp_template_get_base_type (template),
                              gimp_template_get_precision (template),
                              FALSE);
+  gimp_context_set_image (context, image);
 
   gimp_image_undo_disable (image);