ostree_admin_option_context_parse: explicitly clean up when exiting early

ostree_admin_option_context_parse: explicitly clean up when exiting early

The cleanup attribute doesn't clean up before calling a noreturn
function like exit(). Explicitly clean up the pointer variables
(but don't assume that a simple g_object_unref() would be OK either,
in case the behaviour of the cleanup attribute changes).

This isn't a real leak since we're about to exit anyway, but if we
don't fix it then valgrind memcheck will make the tests fail.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #559
Approved by: cgwalters