p_mul: Added checksum from ACP 142(A)

Bug: 10807
Change-Id: Ifd1ea5d67b65a7e7997458785fff95cc471e0dbc
Reviewed-on: https://code.wireshark.org/review/6076
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>