Add validity check to HandleDistanceAndTimeStats_Threaded, to prevent crash

[CTalvio]

Simply adds an IsValid check into the foreach player loop in HandleDistanceAndTimeStats_Threaded.

My servers have been crashing on line 941 of the file as described in #766

Currently testing this fix as a mod, and normally this crash would happen within a couple hours no matter what. One server instance is going on seven hours of matches with no crashes as of yet, but more time would be good. Will report back.

[CTalvio]

Ping to @ASpoonPlaysGames as instructed by contribution guidelines

[ASpoonPlaysGames]

I would prefer if you used a Guard Clause to help readability and reduce the number of lines changed

something like this should be fine:

if ( !IsValid( player ) )
	continue

As for the actual logic behind the change, it looks good. When I wrote this function I was under the (seemingly incorrect) assumption that GetPlayerArray() wouldn't give any invalid players...

It's a bit strange though that the SERVER VM managed to get the "Persistent data not available." error, as there isn't a way for it to check if persistence is available. This leads me to believe that disconnecting the player mid-frame can cause problems, but that's beyond the scope of this fix.

[CTalvio]

I'll make the change once back at my desktop, I've noticed this problem in my mods a couple times where code will try to do things with players that are no longer present, necessitating something like this in some places.

[CTalvio]

Done, also still no crash on my server running the initial fix. I didn't change my test mod to use a guard clause but the effective result should be the same.

[ASpoonPlaysGames]

Code looks good, still confused as to why GetPlayerArray() is giving invalid players but whatever

[GeckoEidechse]

This really isn't testable other than hosting a server for a prolonged period of time and checking for script errors. This is basically what @CTalvio did so I'm forgoing testing on my end ^^