lua: avoid panic if HOOK_GC is not an active hook

Platform panic occurs when fiber.yield() is used within any active
(i.e. being executed now) hook.

It is a regression caused by 96dbc49d
('lua: prohibit fiber yield when GC hook is active').

This patch fixes false positive panic in cases when VM is not running
a GC hook.

Relates to #4518
Closes #5649

Reported-by: Michael Filonenko <filonenko.mikhail@gmail.com>