The shortest valid linkage descriptor contains 56 bits = 7 bytes of payload.
Since the maximum possible payload length is 253 bytes, this leaves 246 bytes
for private data, not 248.

Also, the length checks were changed in order to check the minimum possible
length of the descriptor with the given data, instead of checking maximum
lengths.

(cherry picked from commit 36836f6fec2211d86e62e507570ba7372392d4f6)
Signed-off-by: Jean-Paul Saman <jpsaman@videolan.org>