BUG: IntervalTree should not have NaN in nodes
#### Code Sample, a copy-pastable example if possible
Initializing an `IntervalTree`  with arrays containing `np.nan` can trigger a `RuntimeWarning`:
```python
In [2]: left, right = [0, 1, 2, np.nan], [1, 2, 3, np.nan]

In [3]: tree = pd._libs.interval.IntervalTree(left, right, leaf_size=2)
/home/jeremy/anaconda3/lib/python3.6/site-packages/numpy/lib/function_base.py:4033: RuntimeWarning: Invalid value encountered in median
  r = func(a, **kwargs)
```
This causes some attributes to be incorrectly set as `np.nan`, which can lead to incorrect results:
```python
In [4]: tree.get_loc(0.5)
Out[4]: array([0, 1, 2, 3])
```
#### Problem description

Initializing an `IntervalTree` with data containing `np.nan` triggers a `RuntimeWarning` and can lead to incorrect results.

#### Expected Output
I'd expect no warnings to be raised and for the `get_loc` query to be correct.

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

<details>

INSTALLED VERSIONS
------------------
commit: 437f31cb609372f465aaa853bf4b2299a9550716
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.29-galliumos
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+824.g437f31c
pytest: 3.5.1
pip: 18.0
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
