DEBUG: protocol: yell loudly during registration of invalid sock_domain

The test on the sock_domain is a bit useless because the protocols are
registered at boot time, and the test silently fails and returns no
error. Use a BUG_ON() instead to make sure to catch such bugs in the
code if any.
diff --git a/src/protocol.c b/src/protocol.c
index b33e494..dc0e41d 100644
--- a/src/protocol.c
+++ b/src/protocol.c
@@ -35,12 +35,15 @@
 /* Registers the protocol <proto> */
 void protocol_register(struct protocol *proto)
 {
+	int sock_domain = proto->fam->sock_domain;
+
+	BUG_ON(sock_domain < 0 || sock_domain >= AF_CUST_MAX);
+
 	HA_SPIN_LOCK(PROTO_LOCK, &proto_lock);
 	LIST_APPEND(&protocols, &proto->list);
-	if (proto->fam->sock_domain >= 0 && proto->fam->sock_domain < AF_CUST_MAX)
-		__protocol_by_family[proto->fam->sock_domain]
-			[proto->sock_type == SOCK_DGRAM]
-			[proto->ctrl_type == SOCK_DGRAM] = proto;
+	__protocol_by_family[sock_domain]
+	                    [proto->sock_type == SOCK_DGRAM]
+	                    [proto->ctrl_type == SOCK_DGRAM] = proto;
 	HA_SPIN_UNLOCK(PROTO_LOCK, &proto_lock);
 }