Fix nil error trying to address tPosPlayer after switching zones

Got this error after zoning out of an instance:

...ildStar\Addons\VikingNameplates\VikingNameplates.lua:872: attempt to index global 'tPosPlayer' (a nil value)
stack trace:
	...ildStar\Addons\VikingNameplates\VikingNameplates.lua:872: in function 'CheckDrawDistance'
	...ildStar\Addons\VikingNameplates\VikingNameplates.lua:412: in function 'UpdateNameplateVisibility'
	...ildStar\Addons\VikingNameplates\VikingNameplates.lua:402: in function 'UpdateAllNameplateVisibility'
	...ildStar\Addons\VikingNameplates\VikingNameplates.lua:374: in function <...ildStar\Addons\VikingNameplates\VikingNameplates.lua:373>

Also, make them local.