When reading files from B2 via cloudflare using --b2-download-url
cloudflare strips the Content-Length headers (presumably so it can
inject stuff into the body).
This caused rclone to think the file was corrupted as the length
didn't match.
The patch uses the old length read from the listing if there is no
Content-Length.
See: https://forum.rclone.org/t/b2-cloudflare-error-directory-not-found/23026
d042f3194f