Series.combine with fill_value gives unexpected results
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

a = pd.Series([1, 2, 3], index=["a", "b", "c"])
b = pd.Series([10, 20, 30], index=[0, "e", "f"])
c = b.combine(a, lambda x,y: x+y, fill_value=0)
print(c)
```
#### Problem description
This gives the output:
0    11
a     1
b     2
c     3
e    20
f    30
dtype: int64
#### Expected Output
0    10
a     1
b     2
c     3
e    20
f    30
dtype: int64

With higher numbers in the index it gives expected output i.e.:
```python
a = pd.Series([1, 2, 3], index=["a", "b", "c"])
b = pd.Series([10, 20, 30], index=[4, "e", "f"])
c = b.combine(a, lambda x,y: x+y, fill_value=0)
print(c)
```
4    10
a     1
b     2
c     3
e    20
f    30
dtype: int64
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 44.0.0.post20200106
Cython           : 0.29.14
pytest           : 5.3.2
hypothesis       : 4.54.2
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.11.1
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.2
sqlalchemy       : 1.3.12
tables           : 3.6.1
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
​</details>
