`.loc[] = pd.Series({...})` segmentation fault
#### Code Sample, a copy-pastable example if possible

test.csv:
```
"A","B"
"Давид Юрченко","Tosno"
"Евгений Чернов","Tosno"
"Раде Дугалич","Tosno"
"Виталий Шахов","Tosno"
"Аслан Дудиев","Tosno"
"Вагиз Галиулин","Tosno"
"Нуну Роша","Tosno"
"Георгий Мелкадзе","Tosno"
```

test.py:
```python
import pandas as pd

df = pd.read_csv('test.csv', sep=',').set_index('A', drop=False)

for s in [
'Евгений Чернов',
'Раде Дугалич',
'Виталий Шахов',
'Аслан Дудиев',
'Александр Карницкий',
'Вагиз Галиулин',
'Нолан Ру'
    ]:
    df.loc[s] = pd.Series({ 'A': 'value' })
```

#### Problem description

This code *sometimes* causes a segmentation fault.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.39-calculate
machine: x86_64
processor: Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
byteorder: little
LC_ALL: None
LANG: ru_RU.utf8
LOCALE: ru_RU.UTF-8

pandas: 0.23.0
pytest: None
pip: 9.0.3
setuptools: 39.0.1
Cython: None
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>

cc @TomAugspurger