avformat/utils: Also fill dts==RELATIVE_TS_BASE packets in update_initial_durations()

This dts value can end up in the list in the absence of durations and is in that
case semantically identical to AV_NOPTS_VALUE. We can alternatively prevent
storing RELATIVE_TS_BASE if there is no duration.

Fixes Ticket3640

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
