Revision 1615349

Teach checked svn_mutex__t instances to detect unlock attempts on mutexes
that are not locked.

* subversion/include/svn_error_codes.h
  (SVN_ERR_INVALID_UNLOCK): Declare new error code.

* subversion/libsvn_subr/mutex.c
  (svn_mutex__unlock): Verify that there was actually a lock on this mutex.