Update for compatibility with latest WireShark, and fix MSTNAK handling bug

[dchauran]

While the existing code mostly works in the current version of WireShark there are a few issues that either cause errors or outright break some scenarios.

1. The current version of LUA does not support info(). This was causing failures with MSTNAK packet processing. The fix is to use print().
2. The current version of LUA does not support math.pow(), causing errors in calculating BER. The fix is to use ^ instead.
3. There was a null (nil in LUA) reference issue with MSTNAK packets where we sometimes did not set the message, but then tried to reference it for the info column. The logic has been updated to consistently set this.