backport r1154808 from trunk:

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, sf, trawick



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1164941 13f79535-47bb-0310-9956-ffa450edef68