Use variable name 'len' when handling length.

Change-Id: Icb1d81316d1ead3455c68ad55e52b8120b7dccf9
Reviewed-on: https://code.wireshark.org/review/4609
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>