BUG/MINOR: httpclient: return NULL when no proxy available during httpclient_new()

BUG/MINOR: httpclient: return NULL when no proxy available during httpclient_new()

Latest patches on the mworker rework skipped the httpclient_proxy
creation by accident. This is not supposed to happen because haproxy is
supposed to stop when the proxy creation failed, but it shows a flaw in
the API.

When the httpclient_proxy or the proxy used in parameter of
httpclient_new_from_proxy() is NULL, it will be dereferenced and cause a
crash.

The patch only returns a NULL when doing an httpclient_new() if the
proxy is not available.

Must be backported as far as 2.7.