title: scipy.signal.wiener always show warning
body: <p>I'm trying to use the wiener denoise function from Scipy in a grayscale image:</p>
  
  <pre><code>image = data.imread(image_path, as_grey=True)
  image = signal.wiener(image)
  </code></pre>
  
  <p>but I always get this warning:</p>
  
  <pre><code>/usr/lib/python2.7/dist-packages/scipy/signal/signaltools.py:425: RuntimeWarning: divide by zero encountered in true_divide
        res *= (1 - noise / lVar)
      /usr/lib/python2.7/dist-packages/scipy/signal/signaltools.py:425: RuntimeWarning: invalid value encountered in multiply
        res *= (1 - noise / lVar)
  </code></pre>
  
  <p>The filter works fine, but why I get this warning?</p>