[Bug 765729] qtdemux: Always set a group-id on STREAM_START

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=765729

--- Comment #1 from Edward Hervey <bilboed at bilboed.com> ---
Created attachment 326947
  --> https://bugzilla.gnome.org/attachment.cgi?id=326947&action=edit
qtdemux: Always set a group-id on STREAM_START

If upstream doesn't provide one (or we are pull-based), create one

And while we're at it, just use a local variable

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