Occasional failure to terminate a pipeline with uridecodebin on a matroska file

Submitted by Vincent Penquerc'h `@vincent`

This pipeline terminates about two thirds of the time:

gst-launch-1.0 uridecodebin uri=file:///tmp/mewmew-vorbis-ssa.mkv ! audioconvert ! fakesink

I'm not sure it's matroskademux, as this one seems to always terminate:

gst-launch-1.0 filesrc location=/tmp/mewmew-vorbis-ssa.mkv ! matroskademux ! vorbisparse ! fakesink

However, with an Ogg/Vorbis file, the pipeline always terminates.