Fix segfault in get_wc_or_repos_merge_info() when merging to missing target.
Fix segfault in get_wc_or_repos_merge_info() when merging to missing target.

* subversion/libsvn_client/merge.c
  (get_wc_or_repos_merge_info): Error out when an abbreviated entry for a
  merge target is found, either the target is missing or we have the wrong
  adm access.

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