apr_os_proc_mutex_get() on Unix:  Avoid segfault for cross-
process pthread mutexes.

Submitted by: Yann Ylavic <ylavic.dev gmail.com>
Reviewed by: trawick


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1587063 13f79535-47bb-0310-9956-ffa450edef68