Fix test, also do not read from stack of another thread

Fix test, also do not read from stack of another thread

Summary:
... even though Google's signal handler does it
(https://code.google.com/p/google-glog/source/browse/trunk/src/signalhandler.cc?r=16#235)

Assuming the existence of an invalid pthread_t is a lesser evil than reading
from another thread's stack, IMO.

Test Plan: folly/experimental/symbolizer/test

Reviewed By: simpkins@fb.com

FB internal diff: D1096620