FITS objects created with `fromstring` have problems?

I have some FITS objects being created from strings.  I can't access their `data` attribute.  I think it has to do with `_orig_bitpix` being negative, possibly, but I'm not sure.  @embray any thoughts?  I can provide an example if needed.

```
In [32]: blah._orig_bitpix
Out[32]: -32

In [33]: blah.data
Traceback (most recent call last):
  File "<ipython-input-33-dd8ff7218a27>", line 1, in <module>
    blah.data
  File "/Users/adam/repos/astropy/astropy/utils/misc.py", line 286, in __get__
    val = self._fget(obj)
  File "/Users/adam/repos/astropy/astropy/io/fits/hdu/image.py", line 218, in data
    data = self._get_scaled_image_data(self._data_offset, self.shape)
  File "/Users/adam/repos/astropy/astropy/io/fits/hdu/image.py", line 594, in _get_scaled_image_data
    if self._file.memmap:
AttributeError: 'NoneType' object has no attribute 'memmap'

> /Users/adam/repos/astropy/astropy/io/fits/hdu/image.py(594)_get_scaled_image_data()
    593             else:  # floating point cases
--> 594                 if self._file.memmap:
    595                     data = raw_data.copy()
```
Yeah, but I'm not going to push another commit just to fix a one letter typo in a comment :)  It'll be fixed elsewhen.