fixup check_victory

*** Experimental branch ***

It seems like common sense that the check victory function should be basically symmetric with
regards to host and client, and the current implementation definitely isn't, because it checks
is_observer. This change is a "common sense fix", but not necessitated by any known bugs,
afaik. I believe this version would in all common cases be essentially the same.

However I don't have much appetite to test this change right now, nor do I think it would be
a good idea to merge it to 1.12 without a good reason. So I'm just putting it up as a PR
so that it may perhaps be revisited later.