voip_calls: Regression fixes.

Fix struct initialization logic. Clear a GQueue instead of deleting it.
Don't crash if we have no sequence diagram items. Make sure we show all
flows and not just invites. Zero allocated memory in a couple of places.

Change-Id: Ia5bb3ba57cf625de4b554b354e098aa0361dff28
Reviewed-on: https://code.wireshark.org/review/5390
Reviewed-by: Gerald Combs <gerald@wireshark.org>