Prevent a crash if a non-existent internal RewriteMap is specified

in a server context with RewiteEngine off, then later referenced.

Submitted By: Ben Noordhuis
Reviewed By: covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1154808 13f79535-47bb-0310-9956-ffa450edef68