convolve_fft does not deal with infinity values

Convolve_fft does not deal well with ``inf`` values, and the error message is wrong:

```
In [28]: convolve_fft([1,np.inf,1],[0,1,0])
ERROR: ValueError: Encountered NaNs in convolve.  This is disallowed. [astropy.nddata.convolution.convolve]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-28-d6fa5a67efd2> in <module>()
----> 1 convolve_fft([1,np.inf,1],[0,1,0])

/Users/tom/Library/Python/3.2/lib/python/site-packages/astropy-0.3.dev3556-py3.2-macosx-10.6-x86_64.egg/astropy/nddata/convolution/convolve.py in convolve_fft(array, kernel, boundary, fill_value, crop, return_fft, fft_pad, psf_pad, interpolate_nan, quiet, ignore_edge_zeros, min_wt, normalize_kernel, fftn, ifftn, complex_dtype)
    437     if np.isnan(fftmult).any():
    438         # this check should be unnecessary; call it an insanity check
--> 439         raise ValueError("Encountered NaNs in convolve.  This is disallowed.")
    440 
    441     # restore nans in original image (they were modified inplace earlier)

ValueError: Encountered NaNs in convolve.  This is disallowed.
```

cc @keflavich