#250 melt 6.5.0 hangs on .mp4 video file

[Harald Albrecht]

So my HDMI recorder this day generated an MP4 video file (~6MB) which plays and seeks totally fine in ffplay 3.0.5, but which hangs melt 6.5.0 (master), when seeking around frame 383 or so. A normal play from start to end won't hang melt, but seeking in the video causes melt to stop responding and causing some moderate CPU load. I've only found this because Kdenlive then hangs while seeking in its clip monitor.

OS is Kubuntu 16.10; ffplay is 3.0.5-0ubuntu0.16.10.1 from Ubuntu repos; melt is 6.5.0 from the Kdenlive master ppa.

I will happily supply the problematic video file, but I won't upload it here or to any other public place. Sorry for any inconvenience this may cause.