Fix OpenRA.GetRandomInteger crashing under certain circumstances

OpenRA.GetRandomInteger crashes when the supplied high value is less or
equal to the low value.  A situation where this might occur is when you
try to get a random member of a dynamically generated array that
happens to have only a single member.

mods/common/lua/openra.lua

@@ -43,7 +43,11 @@ OpenRA.SetWinState = function(player, winState)
 end
 
 OpenRA.GetRandomInteger = function(low, high)
+	if high <= low then
+		return low
+	else
 		return Internal.GetRandomInteger(low, high)
+	end
 end
 
 OpenRA.TakeOre = function(player, amount)