Crash when passing a list of illegal values.

``` python
In [3]: intbitset([-2.3], sanity_checks=True)
*** Error in `python_': double free or corruption (fasttop): 0x0000000001291da0 ***
zsh: abort (core dumped)  ipython
```

``` python
In [3]: intbitset([{}], sanity_checks=True)
*** Error in `python_': double free or corruption (fasttop): 0x00000000028d53c0 ***
zsh: abort (core dumped)  ipython
```
