Fix bug 9835 disabled second media stream disables all media streams

Fix bug 9835 disabled second media stream disables all media streams

When a single media line is rejected in an SDP answer, for example a second
'm=video' line, wireshark disables ALL media sessions, instead of just that
one.  But per the RFCs, all it should do is disable just the one RTP media
session the m= line represents. This commit fixes that, so that a disabled
media session (one with a m= port of 0) in the SDP answer only disables its
associated/paired media stream in the offer.

Change-Id: I9bd0d3fc88b8eaa55207c9bf3f3e37da7746fd14
Reviewed-on: https://code.wireshark.org/review/526
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>