[BUG] http: dispatch and http_proxy modes were broken for a long time

[BUG] http: dispatch and http_proxy modes were broken for a long time

Both dispatch and http_proxy modes were broken since 1.4-dev5 when
the adjustment of server health based on response codes was introduced.
In fact, in these modes, s->srv == NULL. The result is a plain segfault.
It should have been noted critical, but the fact that it remained 6
months without being noticed indicates that almost nobody uses these
modes anymore. Also, the crash is immediate upon first request.

Further versions should not be affected anymore since it's planned to
have a dummy server instead of these annoying NULL pointers.