Fix another global pool destruction ordering problem in the bdb code.
Fix another global pool destruction ordering problem in the bdb code.

Previously, we potentially tried to use the bdb cache mutex after its
pool had already been cleared.

Reviewed by: brane
             cmpilato

* subversion/libsvn_fs_base/bdb/env.c
  (clear_cache_lock): New cleanup function, NULLs out the bdb cache lock.
  (svn_fs_bdb__init): Set pool cleanup on the bdb cache pool.
  (acquire_cache_mutex, release_cache_mutex): Don't do anything to the
   lock if it's NULL.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@860789 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion