Slight performance improvement.

Only need to check whether the current program is blocked, since the
other will have switched if it was going to block.

2017/day-18/solution.lua

@@ -117,7 +117,7 @@ end
 while true do
 	instr = instructions[iptr[pid]]
 
-	if waiting[0] and waiting[1] then
+	if waiting[pid] then
 		print("Deadlocked!")
 		break
 	end