BUG: SEGFAULT when assigning nan via iloc
I have a DataFrame that looks like this:

``` python
>>> df.dtypes
ask                 float64
bbg                  object
bid                 float64
last                float64
quotetime            object
size                float64
snap                 object
snaptime            float64
tradedate    datetime64[ns]
tradetime            object
sec_type             object
dtype: object

>>> df.shape
(164618, 11)

df.sec_type.head()
0    Equity
1    Equity
2    Equity
3    Equity
4    Equity
Name: sec_type, dtype: object
```

Now, this segfaults:

``` python
>>> df.sec_type.iloc[0] = np.nan
fish: Job 1, 'ipython --pdb' terminated by signal SIGSEGV (Address boundary error)
```

This also segfaults, but not right away:

``` python
>>> df.sec_type.iloc[0] = None
>>> df.iloc[:10]
fish: Job 1, 'ipython --pdb' terminated by signal SIGSEGV (Address boundary error)
```

And this:

``` python
>>> df.sec_type.ix[0] = np.nan
>>> df.sec_type.head()
fish: Job 1, 'ipython --pdb' terminated by signal SIGSEGV (Address boundary error)
```

However this doesn't:

``` python
s = df.sec_type.copy()
s.iloc[0] = np.nan
```

I tried recreating the error with a minimal case but couldn't since small trivial examples don't seem to segfault; debugging is also kinda hard cause it goes deep into pandas guts..

This happened right after I updated to 0.13, never happened before.

Any ideas?
