Fix reentrancy guard in setDebugTimeLimit()

Summary: $initialized is never initialized and onDebugTick() may be registered multiple times.

Test Plan: None. The function is normally only called once.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25469