Negative indices in unfull buffers unsafely access memory

In a partly-full buffer, if you take a negative index, it will refer that index to the end of the full length of the buffer as apposed to the end of the full portion of the buffer.

```python
from numpy_ringbuffer import RingBuffer
r = RingBuffer(10)
for i in range(5):
    r.append(i)
print(r[-1])
# 3.5097786643241087e+64
```