posts: fix calculation of animated PNG duration.

Fix certain animated PNGs returning NaN as the duration because the
frame rate was being reported as "0/0" by FFMpeg. This happens when the
animation has zero delay between frames. This is supposed to mean a PNG
with an infinitely fast frame rate, but in practice browsers limit it to
around 10FPS. The exact frame rate browsers will use is unknown and
implementation defined.

app/logical/ffmpeg.rb

@@ -59,10 +59,18 @@ class FFmpeg
     end
   end
 
+  # @return [Float, nil] The frame rate of the video or animation, or nil if
+  # unknown. The frame rate can be unknown for animated PNGs that have zero
+  # delay between frames.
   def frame_rate
     rate = video_streams.first[:avg_frame_rate] # "100/57"
     numerator, denominator = rate.split("/")
-    (numerator.to_f / denominator.to_f)
+
+    if numerator.to_f == 0 || denominator.to_f == 0
+      nil
+    else
+      (numerator.to_f / denominator.to_f)
+    end
   end
 
   def video_streams