mount: Corrent sibling path when parent and child have the same mountpoint

When m and pa are mounted in the same directory, rpath is initialized
to empty string (rpath[0] points to '\0'). In this case snprintf() at
the bottom of function makes path have '/' at the end.

In further, function validate_shared(), which uses the result, calls
find_shared_peer() and fails to find a peer.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/path.c

@@ -94,7 +94,8 @@ char *mnt_get_sibling_path(struct mount_info *m,
 	if (rpath[0] == '/')
 		rpath++;
 
-	off = snprintf(path, len, "/%s", rpath);
+	if (rpath[0] != '\0')
+		off = snprintf(path, len, "/%s", rpath);
 
 	return buf;
 }