Don't subtract 4 from the length passed to update_crc10_by_bytes_iuup().

The length passed to it already has 4 subtracted from it by the caller.

Change-Id: I6e047c6c4c4cd5220be923b4663088b6b275d768
Reviewed-on: https://code.wireshark.org/review/3511
Reviewed-by: Guy Harris <guy@alum.mit.edu>