When RTPJitterBuffer resets itself in caluclate_pts(), it returns
GST_CLOCK_TIME_NONE and the packet should be discarded, according
to the behaviour of the standard jitterbuffer.
When encountering an unexpected gap in received seqnums, ips_pts
and ips_rtptime must be reset to avoid calculating a potentially
invalid packet spacing the next time sequential packets are
received.
