actually store the endpoint in the conversation

this fixes the bugfix for
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=10386
(hopefully)

Change-Id: Ib61e1033cae7516181c3557bf78d01b4249a7393
Reviewed-on: https://code.wireshark.org/review/3738
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>