Fix validation of the send_sock in t_uac().

If a send_sock is already provided, no need to do socket discovery again. We need just the protocol (from mk_proxy) in order to validate (or not) the already given send_sock. Do socket discovery only if there no send_sock given or the protocol does not match.

(cherry picked from commit c95ddd8f282c8809a0fae233bdb57b4dddb3c628)