Fix use of default values in state method AddAuraToGUID().

If start or ending were nil parameters to the method, then the arithmetic
used within the method to determine the aura's duration would fail due to
arithmetic on nil values.

Aura.lua

@@ -1469,7 +1469,7 @@ statePrototype.AddAuraToGUID = function(state, guid, auraId, casterGUID, filter,
 	aura.filter = filter
 	aura.start = start or 0
 	aura.ending = ending or INFINITY
-	aura.duration = ending - start
+	aura.duration = aura.ending - aura.start
 	aura.gain = aura.start
 	aura.stacks = 1
 	aura.debuffType = debuffType