fixed:Preview filter closes suddenly: pure virtual method called

[Jan Gruuthuse]

Fixed: d93a317          [flyDialog] better playback

1fcc65f          [core/ffmpeg] Backport nvenc not requiring nonfree flag from ffmpeg 3.1
5c8664c          [flyDialog] proper export
- ubuntu 16.04.1 64-bit Qt5
- ubuntu 14.04.5 64-bit Qt4

video: 720p 3sat
Video Decoder(s):
- VDPAU VDPAU
- Lavcodec Xvideo
Apply filter
[Preview]

When trying to preview avidemux suddenly closes.

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1 active filters
0 Black
Active item :0x372a3d0
[preview] 05:15:43-832  Rank : 0
pure virtual method called
terminate called without an active exception