Fix #193 (use RtlMoveMemory instead of msvcrt.memcpy)

This short patch seems to fix the illegal call of `msvcrt.memcpy` that was introduced between 1.10.1 and 1.11 by replacing it with the underlying `kernel32.RtlMoveMemory` call.

In addition, a little bit of sanity checking is added to make sure writes aren't out of bounds.

Refs paramiko/paramiko#193.

Test case
--------------

This throws an exception on 1.11.0 and subsequent versions, doesn't with this patch applied.

```
from paramiko.agent import Agent
for key in Agent().get_keys():
    print key.get_name(), key.get_base64()`
```