Don't choke when calling gimp_tool_rectangle_set_constraint() while
there's no active image, or while the active image has no active
layer, which can happen when updating the default aspect ratio of
the crop tool.  This would previously result in CRITICALs.

Additionally, use weak pointers for the crop tool's current_image
and current_layer members, to avoid potential dangling pointers.
While not currently necessary, this makes the code less dependent
on the exact order of events.