Fix segfault in svnmucc.

* subversion/libsvn_client/mtcc.c
  (svn_client_mtcc_commit):  We explicitly destroy the MTCC pool and
                             have no knowledge about its relation to
                             SCRATCH_POOL.  Thus, we can't use the
                             for anything non-trivial.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555350 13f79535-47bb-0310-9956-ffa450edef68