TCP: fix wrongly set base_seq when ISN is 0 and tcp_relative_seq is
enabled

tcp_analysis::base_seq could be set several times when the
TCP ISN was set to 0, thus inducing some undesired wraps such as 0-1

Bug: 10713
Change-Id: I69a0dfe677e93bf51015bf7a39ebf888631b12a4
Reviewed-on: https://code.wireshark.org/review/5387
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>