btrfs-progs: handle readdir() end in get_first_device_zone_size()

readdir() could potentially return NULL in case there's no device in
the directory, which is unlikely but should be handled. Reported by
CodeQL scan.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

cmds/filesystem-usage.c

@@ -417,6 +417,8 @@ static u64 get_first_device_zone_size(int fd)
 	}
 	while (1) {
 		de = readdir(dir);
+		if (!de)
+			break;
 		if (strcmp(".", de->d_name) == 0 || strcmp("..", de->d_name) == 0)
 			continue;
 		strcpy(name, de->d_name);