Fixing dot properties access

The commit mishoo/UglifyJS2@753b4b6cc8aff37b9678b3bc3a883874bf4cac51 makes properties names in reserved words accessible with dotted syntax with `screw_ie8` option enabled. However, it also enabled properties names in invalid identifier to be accessible with dotted syntax.

```javascript
// This line:
a["*"] = "asterisk";

// Should be compressed into the following line even with screw_ie8 option enabled:
a["*"]="asterisk";

// Instead it became this, which leads to syntax error:
a.*="asterisk";
```

These commits added scenario in test case to reproduce this issue, and a fix to resolve the problem.