tests: disable part of msg_name test on non-glibc systems

Sadly, musl recvmsg wrapper blindly dereferences 2nd argument,
so limit the affected part of the test to glibc that doesn't.

* tests/msg_name.c (test_msg_name) [!__GLIBC__]: Do not test
with inaccessible 2nd argument.

tests/msg_name.c

@@ -120,9 +120,16 @@ test_msg_name(const int send_fd, const int recv_fd)
 	 * it's ok to fail recvmsg with any reason as long as
 	 * it doesn't read that inaccessible memory.
 	 */
+
+	/*
+	 * Sadly, musl recvmsg wrapper blindly dereferences 2nd argument,
+	 * so limit this test to glibc that doesn't.
+	 */
+#ifdef __GLIBC__
 	rc = send_recv(send_fd, -1, msg + 1, 0);
 	printf("recvmsg(-1, %p, 0) = %d %s (%m)\n",
 	       msg + 1, rc, errno2name());
+#endif
 
 	rc = send_recv(send_fd, -1, 0, 0);
 	printf("recvmsg(-1, NULL, 0) = %d %s (%m)\n",