pipecmd: fix -Wextra complaint about empty body.

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

ccan/pipecmd/pipecmd.c

@@ -123,8 +123,9 @@ pid_t pipecmdarr(int *fd_fromchild, int *fd_tochild, int *fd_errfromchild,
 	child_errno_fail:
 		err = errno;
 		/* Gcc's warn-unused-result fail. */
-		if (write(execfail[1], &err, sizeof(err)))
+		if (write(execfail[1], &err, sizeof(err))) {
 			;
+		}
 		exit(127);
 	}