tight_layout not working with DataFrame.hist
#### Code Sample, a copy-pastable example if possible

First, relevant imports:

``` python
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
```

This doesn't work:

``` python
pd.DataFrame(np.random.randn(100, 3)).hist()
plt.tight_layout()
```

It raises the following error:

```
AttributeError: 'NoneType' object has no attribute 'is_bbox'
```

I suppose this happens because it makes 2x2 plot grid but uses only three subplots. If I use all subplots, then it works:

``` python
pd.DataFrame(np.random.randn(100, 4)).hist()
plt.tight_layout()
```
#### Expected Output

Expected to get tight layout with 2x2 subplot grid.

It should be possible to make this work, because the following works with with matplotlib:

``` python
plt.figure()
plt.subplot(2, 2, 1)
plt.plot([1,2,3])
plt.subplot(2, 2, 2)
plt.plot([4,5,6])
plt.subplot(2, 2, 3)
plt.plot([7,8,9])
plt.tight_layout()
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.4-gnu-1
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_DK.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.1
setuptools: 20.2.1
Cython: 0.24
numpy: 1.11.0
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: 1.4.2
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: None
xlrd: 0.9.4
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.0
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
```
