closure: Prevent use-after-free in closures

Closures trace the function object that they call on, in order to keep
the function alive during garbage collection. When the closure is
invalidated, we break that link so the function can be garbage collected,
but we must do so in an idle function, since it is illegal to stop
tracing a GC-thing in the middle of GC.

However, this caused a possible use-after-free if the closure was
scheduled to stop tracing the function object, but the last reference on
the closure was dropped before the idle function could be run.

Similar to the recent fix in gi/object.cpp [commit 2593d3d4], this avoids
use-after-free by cancelling any pending idle function in the finalize
notifier, and dropping the function object immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=783935