`MultiIndex.get_loc` throws `KeyError` under specific circumstances
#### Code Sample, a copy-pastable example if possible

```python
from pandas import MultiIndex

key = ('a', 7)
idx = MultiIndex(levels=[['a'], [0, 7], [1]],
                 codes=[[0, 0], [1, 0], [0, 0]],
                 names=['x', 'y', 'z'],
                 sortorder=0)

idx.get_loc(key)
```
#### Problem description

Executing the above code results in
```python
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-19-26c5d4d0888a> in <module>
----> 1 idx.get_loc(key)

~/soft/miniconda3/envs/junk/lib/python3.7/site-packages/pandas/core/indexes/multi.py in get_loc(self, key, method)
   2696 
   2697         if start == stop:
-> 2698             raise KeyError(key)
   2699 
   2700         if not follow_key:

KeyError: ('a', 1, 2)
```
Note that the exception disappears if you do any of the following:
- change `sortorder` to `None`
- change the codes of level 1 to `[0, 1]`
- change the values of level 1 to `[0, 7]`
- completely remove level 2

#### Expected Output

I don't think there should be any exception here since the way I created the index appears to be valid, and the `key` is definitely in there.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-23-generic
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           : 0.25.3
numpy            : 1.15.4
pytz             : 2018.9
dateutil         : 2.7.5
pip              : 18.1
setuptools       : 40.6.3
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.2.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.0.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
