protocol: assert if the loop initialization fails on the connection test

This test needs an environment set up to load everything correctly,
running it directly results in a segfault. Let's make sure we assert on
a NULL loop to make this look slightly more planned.

src/modules/module-protocol-native/test-connection.c

@@ -191,6 +191,7 @@ int main(int argc, char *argv[])
 	pw_init(&argc, &argv);
 
 	loop = pw_main_loop_new(NULL);
+	spa_assert_se(loop != NULL);
 	context = pw_context_new(pw_main_loop_get_loop(loop), NULL, 0);
 
 	if (socketpair(AF_UNIX, SOCK_STREAM, 0, fds) < 0) {