Fix crash for group chats (quick and dirty fix sorry)

Hello!

It seems there is a bug in Sigram. Whenever I click on a group chat in the left panel and right before all the conversations appear on the right panel the program crashes.

A quick debug reports that in interface.c:1202 incomingMsg is being called with U=0x0, this is, user_get_chat in the previous line returned 0. I guess this is an unexpected failure? Or should this case be treated separately? It seems those functions "do nothing useful", so I just removed the flow itself.

Thanks!