Avoid some potential segfaults in merge tracking code.
Avoid some potential segfaults in merge tracking code.

* subversion/libsvn_client/merge.c
  (log_find_operative_subtree_revs,
   log_noop_revs,
   log_find_operative_revs): svn_log_entry_receiver_t callbacks may provide
   a svn_log_entry_t with NULL changed_paths2 if the user is not
   authorized to any of the changed paths in a revision.  We obviously don't
   want to try and iterate over a NULL hash, so exit early.

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