Fix undefined attribute when conditional falls through

Trivial change to `wand.image.Image.read`, where an `UnboundLocalError` can be raised on line 1993 due to `r` not being defined due to neither `blob is not None` nor `filename is not None` conditionals being met. Fixed by added `r = None` definition at the top of the method. I did not find another case of this in this file; in each case there was an `else`.