Fix#1768
The `sm_admin`-triggered Menu flow for banning players is caching client
indices inside the basebans.sp `PlayerInfo` struct, and can then
incorrectly use them in the menu callback without checking for the
related client's UserId validity. This leads to bug #1768 occurring when
the ban target player disconnects from the server before the banning
admin could complete the banmenu UI flow.
Since the related menu callbacks can't rely on the cached client index,
I have removed the basebans.sp `PlayerInfo.banTarget` member entirely,
in favor of the `PlayerInfo.banTarget`, and instead call
`GetClientOfUserId(...)` to get & validate the target client index where
necessary. The `PrepareBan(...)` function has been refactored to take a
ban target UserId (instead of the target client index) accordingly.
bf7212889b