Check that V2 Packet Data has length of 1

[bznein]

Surfaced here by @chatton:

I think we want this to be the final state for validate basic for the initial release, but in order for mock applications to be testable with multiple packet datas (which I still think is worthwhile doing ), maybe we can leave this check commented out and create an issue to revert it. (or we could just fail on == 0 but create an issue to change)

[gjermundgaraba]

This issue was mistakenly reopened during some integration work, so closing again now.