mod_proxy: follow up to r1750392.

Avoid double checking the connection in ap_proxy_connect_backend() when
ap_proxy_check_backend() says it is up and good to go.

This can be done by moving the PROXY_WORKER_IS_USABLE() check in
ap_proxy_check_backend(), since it is called by ap_proxy_connect_backend(),
and not calling the latter if the former succeeded (for the modules using it).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1750474 13f79535-47bb-0310-9956-ffa450edef68