Validate client edict bounds/free state in SV_DropClient_PreHook

[gss-crazydog]

GET_PLAYER_POINTER was being called on the raw edict without confirming it was valid.

This would occasionally cause an "Bad entity in IndexOfEdict()" crash after a client disconnected unexpectedly (e.g. a timeout or other network drop).

I extracted a stacktrace from one of these crashes which helped point to this being the cause

...
L 08/08/2026 - 04:22:00: #4 addons/metamod/dlls/metamod.so(+0xa690) [0xe60cc690] mm_Sys_Error(char const*)
L 08/08/2026 - 04:22:00: #5 engine_i486.so(+0x69c4e) [0xee1cdc4e]
L 08/08/2026 - 04:22:00: #6 engine_i486.so(+0x65636) [0xee1c9636]
L 08/08/2026 - 04:22:00: #7 addons/amxmodx/dlls/amxmodx_mm_i386.so(+0x2068e) [0xe546968e] SV_DropClient(client_s*, int, char const*, ...)

I wasn't able to reliably repro this, although it happened relatively frequently, but I have not seen another crash of this type since applying this patch.