subversion: mod_dav_svn assertion when handling certain requests with autoversioning enabled

The get_parent_resource function in repos.c in mod_dav_svn Apache HTTPD server module in Subversion 1.7.11 through 1.7.13 and 1.8.1 through 1.8.4, when built with assertions enabled and SVNAutoversioning is enabled, allows remote attackers to cause a denial of service (assertion failure and Apache process abort) via a non-canonical URL in a request, as demonstrated using a trailing /.