Merge r693727, r693728 from trunk:

* Allow empty substitute patterns (to remove data from the stream), but
  disallow empty search patterns.


* Fix potential segfault if flags remains NULL.

Submitted by: rpluem
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@696321 13f79535-47bb-0310-9956-ffa450edef68