BUG: agg on groups with different sizes fails with out of bounds IndexError
- [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.

---

#### Code Sample, a copy-pastable example
See here as well: https://repl.it/@valkum/WrithingNotablePascal
```python
import numpy as np
import pandas as pd

data = {
  'date': ['2000-01-01', '2000-01-02', '2000-01-01', '2000-01-02'],
  'team': ['client1', 'client1',  'client2', 'client2'],
  'temp': [0.780302, 0.035013, 0.355633, 0.243835],
}
df = pd.DataFrame( data )
df['date'] = pd.to_datetime(df['date'])

df = df.drop(df.index[1])
sampled=df.groupby('team').resample("1D", on='date')
#Returns IndexError
sampled.agg({'temp': np.mean})
#Returns IndexError as well
sampled['temp'].mean()


```

#### Problem description

agg fails with `IndexError: index 3 is out of bounds for axis 0 with size 3`


Note that this does work as expected when I do not drop a row after createing the DataFrame, so I assume it is caused by the index.
#### Expected Output
No fail.
#### Output of ``pd.show_versions()``


<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-1009-gcp
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.17
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
