While our public API does not prohibit nested locking attempts to the
same mutex (e.g. via svn_fs_freeze) and returns an error on them, our
code should always be able to ensure correct lock release.  Therefore,
double release etc. should be considered a malfunction rather than an
ordinary error.

* subversion/include/svn_error_codes.h
  (SVN_ERR_INVALID_UNLOCK): Unreleased and no longer used. Drop.
  (SVN_ERR_INVALID_INPUT): Renumber to use contiguous error codes.

* subversion/libsvn_subr/mutex.c
  (svn_mutex__unlock): Consider improper unlocking a malfunction.

Suggested by: ivan

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618600 13f79535-47bb-0310-9956-ffa450edef68