BUG/MINOR: checks: Set missing id to the dummy checks frontend

The dummy frontend used to create the session of the tcp-checks is
initialized without identifier. However, it is required because this id may
be used without any guard, for instance in log-format string via "%f" or
when fe_name sample fetch is called. Thus, an unset id may lead to crashes.

This patch must be backported as far as 2.2.

src/check.c

@@ -1420,6 +1420,7 @@ static int start_checks()
 
 	/* 0- init the dummy frontend used to create all checks sessions */
 	init_new_proxy(&checks_fe);
+	checks_fe.id = strdup("CHECKS-FE");
 	checks_fe.cap = PR_CAP_FE | PR_CAP_BE;
         checks_fe.mode = PR_MODE_TCP;
 	checks_fe.maxconn = 0;