voip_calls_dialog/voip_calls: Fix for duplicated entries after retap

voip_calls_dialog/voip_calls: Fix for duplicated entries after retap

Issue seen from 2.6:
1) Open pcap with any VoIP call
2) Open Telephony->VoIP calls
3) Select one or more calls
4) Press Flow Sequence - sequence is shown
5) Close Flow Sequence
6) Press Play Streams - Play dialog is shown
7) Close Play dialog
8) Press Flow Sequence - sequence is shown, but all entries are duplicated

If you repeat 6-7 multiple times, all entries are shown multiple times in 8

Patch adds missing clear of graph_analysis before retaping the stream.
Tested on master, but should be backported to as many stable branches as possible.

Change-Id: I9793f6e874defde2f377732d78689e957df71b33
Reviewed-on: https://code.wireshark.org/review/35672
Petri-Dish: Jim Young <jim.young.ws@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>