Fix hasRest to not try to load "-1" from params array. (#6581)

Similar in spirit to https://github.com/babel/babel/pull/6580, the
current implementation did

```js
node.params[node.params.length - 1]
```

where `node.params` can also be empty, which causes it to lookup the
property `"-1"`, which is not found on the array itself and obviously
also not in the `Object.prototype` and the `Array.prototype`. However
since `"-1"` is not a valid array index, but has a valid integer
representation, this is a very expensive lookup in V8 (and probably
other engines too, but that is probably less relevant, since Babel
most often runs on Node nowadays). In V8 this causes a call to
the `%SetProperty` runtime function for each of these `"-1"`
property lookups, and in addition sends the whole `KeyedLoadIC`
to `MEGAMORPHIC` state, which also penalizes other accesses
on this line.

This is a small non-breaking performance fix.

packages/babel-plugin-transform-es2015-parameters/src/rest.js

@@ -155,7 +155,8 @@ const memberExpressionOptimisationVisitor = {
   },
 };
 function hasRest(node) {
-  return t.isRestElement(node.params[node.params.length - 1]);
+  const length = node.params.length;
+  return length > 0 && t.isRestElement(node.params[length - 1]);
 }
 
 function optimiseIndexGetter(path, argsId, offset) {