app: guard against crash due to quitting while DND is processed

In gimp_display_shell_drop_uri_list(), shell->display is dereferenced in
some places without checking that it's still there. It can be set to
NULL if the user quits the application while a drag and drop action is
being processed and the main loop is iterated during execution of this
function. (Bug #652280)
(cherry picked from commit b1a2c736)

Conflicts:

	app/display/gimpdisplayshell-dnd.c