Fix the BDB regression tests with svn_mutex__t.
Fix the BDB regression tests with svn_mutex__t.

* subversion/libsvn_fs_base/bdb/env.c
  (clear_cache): Null the mutex to prevent attempts to use it after
   it has been destroyed.
  (svn_fs_bdb__close): Add comment.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion