MINOR: listener: Remove useless checks on the receiver protocol existence

MINOR: listener: Remove useless checks on the receiver protocol existence

The receiver protocol is always set when a listener is created or cloned. At
least for now. And there is no check on it at many places, except in
listener_accept() function. So, let's remove remaining useless checks. That
will avoid false Coverity reports in future.

This patch should fix the issue #2631.