use packet-scoped memory for the unmasked data tvb

Change-Id: I5f9b769e26c16072a1656dfe9094487d6dfe76f4
Reviewed-on: https://code.wireshark.org/review/6142
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>