BUG: stktable: tcp_src_to_stktable_key() must return NULL on invalid families

Source addresses of non-TCP families were not correctly handled by
tcp_src_to_stktable_key() as it forgot to return NULL and instead left
the previous value in the stick-table buffer.

This bug is 1.5-specific and was introduced by commit 4f92d320 in 1.5-dev6
so it does not need any backport.

include/proto/proto_tcp.h

@@ -52,6 +52,8 @@ static inline struct stktable_key *tcp_src_to_stktable_key(struct session *s)
 	case AF_INET6:
 		static_table_key.key = (void *)&((struct sockaddr_in6 *)&s->si[0].addr.from)->sin6_addr;
 		break;
+	default:
+		return NULL;
 	}
 	return &static_table_key;
 }