Fix a server SEGV when AuthzForceUsernameCase is used without authentication.
Fix a server SEGV when AuthzForceUsernameCase is used without
authentication.

* subversion/mod_authz_svn/mod_authz_svn.c
  (get_username_to_authorize): Allow NULL username.

Patch by: roderich.schupp{_AT_}googlemail.com

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