If parsing an all-zeros checksum, just return NULL, which by definition represents the same thing (but is easier to check for).
If parsing an all-zeros checksum, just return NULL, which by definition
represents the same thing (but is easier to check for).

* subversion/libsvn_subr/checksum.c
  (svn_checksum_parse_hex): Check for the all-zeros parsed checksum.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion