mint-arena: Make bots stop attacking player after disconnect

Bots did not notice player disconnected, so they kept attacking
the last known position.

Checking in BotEntityVisible might fix other similar issues too.
mint-arena: Don't use uninitialized ps from BotAI_GetPlayerState

If BotAI_GetPlayerState returns qfalse, ps is untouched and in
some cases means uninitialized. So don't use it if not valid.