Corrected to use packet_list function argument

[piersoh]

Minor correction so that the internal _write_to_pcap function uses its argument packet_list when calling wrpcap(filename, packets_list) as opposed to the outer scope offline variable.

[gpotter2]

Thanks for the PR. This makes sense, however this will conflict with #3055 (we've ended up refactoring the whole thing).

Maintainers, please do not merge