mod_info: Fix output of server settings for

PIPE_BUF in mod_info in the rare case that
PIPE_BUF is defined.

Backport of 1857515 from trunk.
Proposed by: rjung
Backported by: rjung
Reviewed by: rjung, jailletc36, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1859375 13f79535-47bb-0310-9956-ffa450edef68