rtpbasedepayload: Drop gap events before first buffer

Submitted by Stian Selnes @stianse

Link to original bug (#773104)

Description

Before a gap event is pushed downstream a segment event must be pushed
since the gap event can cause packet concealment downstream and hence
data flow. Since concealment before receiving any data packets usually
doesn't make any sense, the gap event is not sent downstream.

Alternatively one could generate a default caps and segment event, but
no need to complicate things until it's proven necessary.