Fix issue 3237 which relative externals may core dump.
Fix issue 3237 which relative externals may core dump.

* subversion/libsvn_client/externals.c
  (resolve_relative_external_url):
    When resolving a relative URL, if either the parent directory
    URL's or the repository root URL is the server root URL and if the
    URL has no / after the hostname then apr_uri_parse() will leave
    the path as NULL.  So if it's null, set the PATH to /.

Suggested by: julianfoad

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