Follow-up to r1592640: In non-threaded mode, an erroneous attempt
on recursive locking  shall not leave the mutex in an invalid state.

* subversion/libsvn_subr/mutex.c
  (svn_mutex__lock): If the lock "failed", don't update the lock count.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592763 13f79535-47bb-0310-9956-ffa450edef68