Small corrections when setting 'L->top'

- OP_NEWTABLE can use 'ra + 1' to set top (instead of ci->top);
- OP_CLOSE doesn't need to set top ('Protect' already does that);
- OP_TFORCALL must use 'ProtectNT', to preserve the top already set.
  (That was a small bug, because iterators could be called with
   extra parameters besides the state and the control variable.)
- Comments and an extra test for the bug in previous item.

lparser.c

@@ -694,9 +694,10 @@ static Proto *addprototype (LexState *ls) {
 
 /*
 ** codes instruction to create new closure in parent function.
-** The OP_CLOSURE instruction must use the last available register,
+** The OP_CLOSURE instruction uses the last available register,
 ** so that, if it invokes the GC, the GC knows which registers
 ** are in use at that time.
+
 */
 static void codeclosure (LexState *ls, expdesc *v) {
   FuncState *fs = ls->fs->prev;