Add syntax error for incomplete global variables

Same as: https://github.com/mruby/mruby/pull/1136

### Ruby(1.9.3)
```bash
$ ruby -e "p $"
-e:1: syntax error, unexpected $undefined, expecting $end
```

### mruby(current)
```bash
$ mruby -e "p $"
nil
```

### mruby(fixed)
```bash
$ mruby -e "p $"
-e:1:3: incomplete global variable syntax
```