BUG/MINOR: hlua_fcn/queue: fix broken pop_wait()

BUG/MINOR: hlua_fcn/queue: fix broken pop_wait()

queue:pop_wait() was broken during late refactor prior to merge.
(Due to small modifications to ensure that pop() returns nil on empty
queue instead of nothing)

Because of this, pop_wait() currently behaves exactly as pop(), resulting
in 100% active CPU when used in a while loop.

Indeed, _hlua_queue_pop() should explicitly return 0 when the queue is
empty since pop_wait logic relies on this and the pushnil should be
handled directly in queue:pop() function instead.

Adding some comments as well to document this.