Fix alliedmodders#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 alliedmodders#1768 occurring when
the ban target player disconnects from the server before the banning
admin could complete the banning Menu UI flow.

Since the related Menu callbacks can't rely on the client index, I have
removed the basebans.sp `PlayerInfo.banTarget` member entirely, in favor
of the `PlayerInfo.banTargetUserId`, and instead of
`GetClientOfUserId(...)` to get & validate the client index where necessary. The `PrepareBan(...)` function has been refactored to take a ban target UserId (instead of the client index) accordingly.

Fix alliedmodders#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 alliedmodders#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 client index, I have
removed the basebans.sp `PlayerInfo.banTarget` member entirely, in favor
of the `PlayerInfo.banTargetUserId`, and instead call
`GetClientOfUserId(...)` to get & validate the client index where
necessary. The `PrepareBan(...)` function has been refactored to take a ban target
UserId (instead of the client index) accordingly.

Fix alliedmodders#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 alliedmodders#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.