BGP: issue with BGP AS path heuristic (AS_CONFED_SET)

Add also a expert info when unable determine ASN length (2 or 4 bytes)

Bug: 10399
Change-Id: I24978e29e24f38c2e01e4b953a5a51496f0cf5a6
Reviewed-on: https://code.wireshark.org/review/3831
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit d127b573116f04352e811591644e15b69f0f7a62)
Reviewed-on: https://code.wireshark.org/review/3839