Fix a signed/unsigned comparison that can never match.

Fix a signed/unsigned comparison that can never match.

-1 is a valid length value (for socket, pipe and cgi buckets for example)
All path I've checked cast the -1 to (apr_size_t) in order for the comparison to work. So do it as well here.

This has been like that in trunk since r708144, about 11 years ago, so I assume that it is not really an issue.

Spotted by gcc 9.1 and -Wextra

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1864868 13f79535-47bb-0310-9956-ffa450edef68