MINOR: frontend: switch to conn_get_{src,dst}() for logging and debugging

MINOR: frontend: switch to conn_get_{src,dst}() for logging and debugging

The frontend accept code uses conn_get_{from,to}_addr for logging and
debugging, without really checking if the operation succeeds. In
preparation of future changes, let's switch to conn_get_{src,dst}() and
integrate status check for possible failures.