cancellable: Don't assert if finalization races with cancellation

Commit 281e3010

 narrowed the race between GCancellable::cancelled and
GCancellableSource's finalize(), but did not prevent it: there was
nothing to stop cancellation from occurring after the refcount drops
to 0, but before g_source_unref_internal() bumps it back up to 1 to
run finalize().

GCancellable cannot be expected to detect that situation, because the
only way it has to detect last-unref is finalize(), but in that
situation finalize() hasn't happened yet.

Instead of detecting last-unref, relax the precondition a little
to make it detect finalization: priv is only poisoned (set to NULL)
after the finalize() function has been called, so we can assume that
GCancellable has already seen finalize() by then.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=791754
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=884654