fix: consider a connection closed in poll() only if it's not ready too

fix: consider a connection closed in poll() only if it's not ready too

When closing the proxy and trying to communicate, poll returns state 25
on the connection, i.e. POLLIN | POLLERR | POLLHUP. The connection is
not closed though, so we might end up in weird state ahead (e.g. connection
stuck in ACTIVE).

Because it's suggested that the connection is readable, do read from it.
Likely a proper error will be raised downstream, but we will remain in
consistent state.