baseparse: push pending events before GAP event

baseparse: push pending events before GAP event

A GAP event is handled as an empty buffer by sinks and they expect
to receive start up events before GAP events (like a segment).

This is important specially if there is a GAP at the beginning of
a stream (before any buffers) so that the segment event can be
pushed downstream before the GAP

https://bugzilla.gnome.org/show_bug.cgi?id=721350