ipsec: prohibit unknown directions in key_havesp

Eliminates a branch checking for its validity.

Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D36485

sys/netipsec/key.c

@@ -806,8 +806,9 @@ int
 key_havesp(u_int dir)
 {
 
-	return (dir == IPSEC_DIR_INBOUND || dir == IPSEC_DIR_OUTBOUND ?
-		TAILQ_FIRST(&V_sptree[dir]) != NULL : 1);
+	IPSEC_ASSERT(dir == IPSEC_DIR_INBOUND || dir == IPSEC_DIR_OUTBOUND,
+		("invalid direction %u", dir));
+	return (TAILQ_FIRST(&V_sptree[dir]) != NULL);
 }
 
 /* %%% IPsec policy management */