pipes: Add test that pipe data is restored without errors

This eliminates the warning below as well

 | pipes.c: In function ‘open_pipe’:
 | pipes.c:252:6: error: variable ‘ret’ set but not used [-Werror=unused-but-set-variable]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

pipes.c

@@ -266,6 +266,8 @@ static int open_pipe(struct file_desc *d)
 	}
 
 	ret = restore_pipe_data(pfd[1], pi);
+	if (ret)
+		return -1;
 
 	sock = socket(PF_UNIX, SOCK_DGRAM, 0);
 	if (sock < 0) {