BUG: ser.loc[-1] and ser.loc[[-1]] with UInt64Index
```
umax = np.iinfo(np.uint64).max
ser = pd.Series([0, 1], index=[umax-1, umax])

>>> ser.loc[-1]  # <- should raise KeyError
OverflowError: can't convert negative value to npy_uint64

>>> ser.loc[[-1]]  # <- should raise KeyError
18446744073709551615    1
dtype: int64
```