On the invoke-diff-cmd branch:  Cast NULL to char*.

* subversion/libsvn_subr/io.c 

  (svn_io_run_external_diff): Cast Null to char*.




git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1488538 13f79535-47bb-0310-9956-ffa450edef68