On the authzperf branch: Maintain stringbuf invariants.

* subversion/libsvn_subr/string.c
  (svn_stringbuf_leftchop): Make sure that the resulting
   stringbuf is always NUL-terminated.

* subversion/tests/libsvn_subr/string-test.c
  (test_stringbuf_leftchop): New test case.
  (test_funcs): Update test list.

Found by: ivan


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/authzperf@1621178 13f79535-47bb-0310-9956-ffa450edef68