BGP: Incorrect decoding AS numbers when mixed AS size

Fix AS Path Heuristic

Issue reported by Jon

Bug: 10742
Change-Id: Ie5e4108bd93464a2d1076dcc4f322171ea8e68cb
Reviewed-on: https://code.wireshark.org/review/5564
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit f260f9aa2d625bbd592dbaa3ef5a09dbf6c5f747)
Reviewed-on: https://code.wireshark.org/review/5567