When applying the Speed effect to certain videos, the video preview/render goes totally black.

If I re-render the video using kdenlive and I apply the effect on the resulting video it works

I've tried 17.12 and 18.04 versions and both are affected.



Input video
-----------
Input #0, matroska,webm, from 'Screencast_10-05-18_13:00:01.webm':
  Metadata:
    encoder         : GStreamer matroskamux version 1.14.0
    creation_time   : 2018-05-10T11:00:03.000000Z
  Duration: 00:04:17.05, start: 0.019000, bitrate: 65 kb/s
    Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv), 831x757, SAR 1:1 DAR 831:757, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      title           : Video



Same input video re-rendered with an extra audio track
------------------------------------------------------
Input #0, matroska,webm, from '../Documents/untitled.webm':
  Metadata:
    ENCODER         : Lavf57.83.100
  Duration: 00:05:03.00, start: 0.000000, bitrate: 439 kb/s
    Stream #0:0: Video: vp8, yuv420p(bt709/unknown/bt709, progressive), 1920x1080, SAR 1:1 DAR 16:9, 1k fps, 25 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      DURATION        : 00:05:03,003000000
    Stream #0:1: Audio: vorbis, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:05:02,956000000