thread_pthread.c: allocate sigaltstack before pthread_create

A new (not-initialized-yet) pthread attempts to allocate sigaltstack by
using xmalloc.  It may cause GC, but because the thread is not
initialized yet, ruby_native_thread_p() returns false, which leads to
"[FATAL] failed to allocate memory" and exit.

In fact, we can observe the error message in the log of OpenBSD CI:
https://rubyci.org/logs/rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20200306T083005Z.log.html.gz

This changeset allocates sigaltstack before pthread is created.

0 files changed, 0 insertions, 0 deletions