BUG: indexing with -1 doesn't work anymore in expanding function
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
# Your code here
import pandas as pd
import numpy as np
# setup
df = pd.DataFrame([5,7,4,12,3,4,1], columns=['Value'])
# calculate countif
df['Count'] = df.Value.expanding(1).apply(lambda x: np.sum(np.where(x > x[-1], 1, 0))).astype('int')
```
raises `Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/window/expanding.py", line 152, in apply
    return super().apply(func, raw=raw, args=args, kwargs=kwargs)
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/window/rolling.py", line 1300, in apply
    return self._apply(
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/window/rolling.py", line 507, in _apply
    result = calc(values)
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/window/rolling.py", line 495, in calc
    return func(x, start, end, min_periods)
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/window/rolling.py", line 1326, in apply_func
    return window_func(values, begin, end, min_periods)
  File "pandas/_libs/window/aggregations.pyx", line 1375, in pandas._libs.window.aggregations.roll_generic_fixed
  File "<stdin>", line 2, in <lambda>
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/series.py", line 871, in __getitem__
    result = self.index.get_value(self, key)
  File "/home/tobias/anaconda3/envs/py38/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 4405, in get_value
    return self._engine.get_value(s, k, tz=getattr(series.dtype, "tz", None))
  File "pandas/_libs/index.pyx", line 80, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 90, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 138, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 998, in pandas._libs.hashtable.Int64HashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1005, in pandas._libs.hashtable.Int64HashTable.get_item
KeyError: -1
`

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

#### Expected Output

On 0.25.2 this resulted in

```
    Value   Count
0   5        0
1   7        0
2   4        2
3   12       0
4   3        4
5   4        3
6   1        6
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.10.0-35-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : de_DE.UTF-8
LOCALE           : de_DE.UTF-8

pandas           : 1.0.5
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.10.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.3.3
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
