pipecmd: fix warn-unused-result warnings (-O2)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/pipecmd/_info

@@ -28,7 +28,8 @@
  *	char input[12];
  *
  *	if (argc == 2) {
- *		write(STDOUT_FILENO, "hello world\n", 12);
+ *		if (write(STDOUT_FILENO, "hello world\n", 12) != 12)
+ *			exit(1);
  *		exit(0);
  *	}
  *	child = pipecmd(&outputfd, NULL, NULL, argv[0], "ignoredarg", NULL);

ccan/pipecmd/pipecmd.c

@@ -122,7 +122,9 @@ pid_t pipecmdarr(int *fd_fromchild, int *fd_tochild, int *fd_errfromchild,
 
 	child_errno_fail:
 		err = errno;
-		write(execfail[1], &err, sizeof(err));
+		/* Gcc's warn-unused-result fail. */
+		if (write(execfail[1], &err, sizeof(err)))
+			;
 		exit(127);
 	}