Test the behaviour of svn_checksum_match.

* subversion/tests/libsvn_subr/checksum-test.c
  (zero_match): New test.
  (test_funcs): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1457064 13f79535-47bb-0310-9956-ffa450edef68