When using M:N threads, EC is set to NULL in the shared native thread
when nothing is scheduled. This previously caused a segfault when we try
to examine the EC.

Returning 0 instead means we may miss profiling information, but a
profiler relying on this isn't thread aware anyways, and observing that
"nothing" is running is probably correct.

Fixes [Bug #20017]

Co-authored-by: Dustin Brown <dbrown9@gmail.com>