segmentation fault for buffer.copy() with no args

To reproduce from the command line:

```
$ iojs -e "Buffer(10).copy()"
[1]    87376 segmentation fault  iojs -e "Buffer(10).copy()"
```

While this isn't valid input to `copy`, this should probably not segfault.

Tested on iojs 1.8.1. Same issue in node 0.12.2.
