multiqueue: avoid signaling overrun on the first segment

When the first segment has position != 0 and position > max-size-time
it will immediatelly cause the multiqueue to signal overrun.

This can happen easily with adaptive streams when switching bitrates
and starting a new group. The segment for this new group will have
a position that is much greater than 0 and will lead to this issue.
This is particularly harmful when the adaptive stream uses mpegts
that doesn't emit no-more-pads and it might happen that only one
of the stream pads was added when the multiqueue overruns and gets
the group ready for exposing. So the user will only get audio or
video.

The solution is to fallback to the sink segment while the source pad
has no segment.

https://bugzilla.gnome.org/show_bug.cgi?id=729124