ZStatSubPhase::register_start should not call register_gc_phase_start if ZAbort::should_abort()

ZStatSubPhase::register_start should not call register_gc_phase_start if ZAbort::should_abort() is true. This will cause an unbalanced push and pop behaviour of the phase stack as ZStatSubPhase::register_end stops popping (and sending events) after the aborting has started. This will create an issue if more subsequent sub-phases are added in-between two abort points as the phase stack may overflow.

Simply avoid pushing new phases when aborting has started solves this issue.