lib: smb_threads: fix access before init bug

lib: smb_threads: fix access before init bug

talloc_stackframe_internal() calls SMB_THREAD_GET_TLS(global_ts)  which
calls smb_get_tls_pthread() in the POSIX pthread wrapper implementation.

If SMB_THREAD_SET_TLS() hasn't been called before, global_ts is NULL and
smb_get_tls_pthread dereferences it so it crashes.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13505

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit e311801e0e7171a2b50e39d3e0c2d2137f8b3d7e)