Fix a crash during init in libsvn_repos with pool debugging enabled.
Fix a crash during init in libsvn_repos with pool debugging enabled.

* subversion/libsvn_repos/authz.c
  (synchronized_authz_initialize): With APR_POOL_DEBUG, the function
   apr_pool_allocator_get() will return NULL (there is no global allocator).
   So dereferencing that pointer to find the allocator's mutex will segfault.
   It is unclear to me if using apr_allocator_mutex_get() is a sane way to find
   out if we're running in multithreaded mode, and I can't find a better way
   right now. For now, just assume that we're multithreaded if APR has threads.
   This should be safe and allows me to run the tests over DAV again.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1782614 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion