In the fix to Trac #13615 we introduced some logic to atomically blackhole AP_STACKs
closures upon entry. However, this logic was placed *after* a stack pointer
adjustment. This meant that if someone else beat us to blackholing the AP_STACK
we would suspend the thread with uninitialized content on the stack. This would
then later blow up when threadPaused attempted to walk the stack, hence Trac #13970.
Silly bug but still cost lots of head-scratching to find.
Thanks to albertov for the great repro.