Follow-up to r39837.
Follow-up to r39837.

Fixes a slew of merge tests, except merge_tests.py 76, which I'm still
working on.

* subversion/libsvn_client/merge.c
  (do_file_merge): svn_client__merge_path_t member PATH cannot be NULL,
   particularly since r39837 actually uses it!

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