The shell_ready_for_quit() could be called twice, the first time for user-initiated
quit, the second time when the GApplication "shutdown" signal had been invoked. This
invocation had been added recently, to fix memory leaks. While it didn't matter
for desktop environments where the GtkApplication's inhibit/uninhibit didn't work,
it did matter for the GNOME Shell. The second call to shell_ready_for_quit() had been
called inside EShell's finalize, which didn't find the right GtkApplication class (a
NULL had been passed to the function), which caused the crash. The fix is to not call
shell_prepare_for_quit() from within the "shutdown" signal callback when the quit
had been already acknowledged.
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