Fix rewrite io.input inside io.lines.

```
do
	local t = os.tmpname()
	local f = io.open(t, 'w')
	f:write('test')
	f:close()

	local i = io.input()
	for l in io.lines(t) do end
	local n = io.input()
	assert(n == i, tostring(n).." ~= "..tostring(i))
	os.remove(t)
	print('+')
end
```

src/core/org/luaj/vm2/lib/IoLib.java

@@ -371,7 +371,7 @@ public class IoLib extends TwoArgFunction {
 
 	//	io.lines(filename) -> iterator
 	public Varargs _io_lines(String filename) {
-		infile = filename==null? input(): ioopenfile(FTYPE_NAMED, filename,"r");
+		File infile = filename==null? input(): ioopenfile(FTYPE_NAMED, filename,"r");
 		checkopen(infile);
 		return lines(infile);
 	}