Fix issue #3999 'svn merge with bad option or target segfaults'.
Fix issue #3999 'svn merge with bad option or target segfaults'.

* subversion/libsvn_client/cmdline.c
  (check_root_url_of_target): Instead of segfaulting, do as the doc string
   promises: "If a root url is not found for TARGET because it does not
   exist in the repository, then return with no error".

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