Prevent SV_DropClient() from being called recursively

[smallmodel]

This fixes an issue where SV_DropClient() would get called twice on the same client if the later gets kicked and has too many reliable commands right while sending a server command in SV_DropClient().
For example when kicking a client, SV_DropClient() gets called and sends server commands (print + disconnect). If the kicked client has too many reliable commands, SV_AddServerCommand() would call SV_DropClient() again on the same client.

This bug can occur as follow:

• SV_CheckTimeouts() is called
  • SV_DropClient() gets called on a client that timed out
    • SV_SendServerCommand() gets called to print stuff
      • SV_AddServerCommand() is called on the client being dropped
        • SV_DropClient() is called because it's exactly the 513th command
          • Print stuff, call VM stuff, send "disconnect", reset the user info...
    • Call VM stuff, send "disconnect", reset the user info...

[timangus]

Good catch.