Fix invalid IsGUIDInGroup call

[Exoridus]

I have noticed some Lua errors getting thrown by Killtrack since the latest wrath patch.
It seems that this is caused due IsGUIDInGroup() not accepting invalid GUIDs anymore instead of failing gracefully.

Stack trace from Bugsack:

162x KillTrack/KillTrack.lua:337: Usage: IsGUIDInGroup(guid[, groupType])
[string "=[C]"]: in function `IsGUIDInGroup'
[string "@KillTrack/KillTrack.lua"]:337: in function `IsInGroup'
[string "@KillTrack/KillTrack.lua"]:228: in function `?'
[string "@KillTrack/KillTrack.lua"]:116: in function `OnEvent'
[string "@KillTrack/KillTrack.lua"]:737: in function <KillTrack/KillTrack.lua:737>

Locals:
(*temporary) = "<No One>"

[Sharparam]

Nice catch! The drawback of testing on an empty PTR server with no other players around :)