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.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@875013 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion