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.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1182492 13f79535-47bb-0310-9956-ffa450edef68

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