dont try to extract frames from audio

pandora/item/models.py

@@ -1394,6 +1394,7 @@ class Item(models.Model):
         else:
             if 'videoRatio' in self.json and self.sort.duration:
                 width, height = self.json['resolution']
+                if width and height:
                     pos = self.sort.duration / 2
                     for p in map(int, [pos/2, pos, pos+pos/2]):
                         path = self.frame(p, height)