Server crash due to _stats.nut (error: Persistent data not available.)

[CTalvio]

I've identified one way a server can crash, this seems to currently be the most common cause for my servers crashing.

The logs will look something like this, and are preceded by the relevant lines about a player disconnecting.

[16:29:08] [SCRIPT SV] [info] [BTB] AFK player kicked: playername - PID
[16:29:08] [NORTHSTAR] [info] Player playername disconnected: "You were disconnected due to being AFK"
[16:29:08] [NORTHSTAR] [info] Player playername disconnected: "You were disconnected due to being AFK"
[16:29:08] [NORTHSTAR] [warn] attempted to write pdata of size 0!
[16:29:08] [SCRIPT SV] [info] SCRIPT ERROR: [SERVER] Persistent data not available.
[16:29:08] [SCRIPT SV] [info]  -> return expect float( player.GetPersistentVar( statString ) )
[16:29:08] [SCRIPT SV] [info]
CALLSTACK
*FUNCTION [GetPlayerStatFloat()] sh_stats.gnut line [469]
*FUNCTION [StatsCallback_ItemUnlockUpdate()] _items.nut line [10171]
*FUNCTION [Stats_RunCallbacks()] mp/_stats.nut line [311]
*FUNCTION [Stats_IncrementStat()] mp/_stats.nut line [298]
*FUNCTION [HandleDistanceAndTimeStats_Threaded()] mp/_stats.nut line [941]


abnormal program termination

So I went down the callstack, and tried adding a validity check before the section of code that seems to be causing this.

Relevant PR once I make it: #767