gh-157639: Fix use-after-free when an external timer re-enters the profiler - #157648

flush_unmatched() calls Stop() on a context before freeing it, and Stop() runs the external timer while that context is still the current one. A timer that calls enable() re-arms sys.monitoring mid-call, so the unmatched C return that follows reaches ptrace_leave_call() and pushes the context onto the free list. flush_unmatched() then frees it, and the next ptrace_enter_call() reads the dangling pointer.

POF_EXT_TIMER already exists to keep the external timer from reaching the profiler's state, so honour it in the two trace callbacks and drop events raised while the timer runs.

Fixes #157639

• Issue: heap uaf in _lsprof via enable() #157639