mod_proxy: Fix ProxySourceAddress binding failure with AH00938. PR 56687.

Proposed by: Arne de Bruijn <apache arbruijn.dds.nl
Reviewed by: ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1703902 13f79535-47bb-0310-9956-ffa450edef68