add the bus_id when we search for an existing conversation to our endpoint
(the existing code uses bus_id 0 by default and creates a new conversation)

Change-Id: I33a105bdf4218ac2dd21cf76825e972ff2dd1b9b
Reviewed-on: https://code.wireshark.org/review/3583
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>