* app/gimpdrawable.c: gimp_drawable_get_color_at() now silently
returns NULL again if the coordinates are out of range. A lot of
code using this function relies on this feature and correctly
checks the return value. No need to emit critical warnings here.

The GTK_CHECK_TYPE macro test for obj != NULL, no need to do this
check twice. Removed lots of unnecessary calls to g_return_if_fail().

* app/color_picker.c: with the old behaviour of
gimp_drawable_get_color_at() the code is a bit simpler.

* app/fuzzy_select.c: fuzzy_select relied on drawable_offsets()
returning off_x = off_y = 0 if drawable == NULL. Decided to change
this here, fixes bug #7077.

* app/gimpimage.[ch]: Even though we made bad experiences with the
changes in gimpdrawable.c, I have introduced similar argument checks
here.

* app/image_map.c: indentation


--Sven