core: speed up (for common cases) and reduce memory usage of ap_escape_logitem

This should save 70-100 bytes in the request pool for a default config.

trunk patch: http://svn.apache.org/r1485409
Submitted by: jailletc36
Reviewed by: jim, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1485723 13f79535-47bb-0310-9956-ffa450edef68