qtdemux: Handle empty segments when seeking in reverse play.
qtdemux: Handle empty segments when seeking in reverse play.

Empty segments in an edit list have a media_start time of -1,
as they don't actually play any media. Allow for that when
aligning to the reference stream in reverse play.

Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,kittee/gst-plugins-good,chamois94/gst-plugins-good,BigBrother-International/gst-plugins-good,loshca/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,BigBrother-International/gst-plugins-good,chamois94/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,veo-labs/gst-plugins-good,loshca/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,BigBrother-International/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,ijsf/OpenWebRTC-gst-plugins-good,kittee/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,loshca/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,vatavuserban/gst-plugins-good,BigBrother-International/gst-plugins-good,loshca/gst-plugins-good,sebras/gst-plugins-good,sh0/gst-plugins-good,rawoul/gst-plugins-good,sh0/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good,GrokImageCompression/gst-plugins-good,kittee/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,veo-labs/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,ikonst/gst-plugins-good,sh0/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,Kurento/gst-plugins-good,froggatt/gst-plugins-good-m,sh0/gst-plugins-good,Kurento/gst-plugins-good,vatavuserban/gst-plugins-good,GStreamer/gst-plugins-good,froggatt/gst-plugins-good-m,ikonst/gst-plugins-good,pexip/gst-plugins-good,shelsonjava/gst-plugins-good,veo-labs/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,kittee/gst-plugins-good,rawoul/gst-plugins-good,veo-labs/gst-plugins-good,Kurento/gst-plugins-good