lavf/mov.c: Use the first sidx for tracks without sidx.

According to spec ISO_IEC_15444_12 "For any media stream for which no segment index is present, referred to as non‐indexed stream, the media stream associated with the first Segment Index box in the segment serves as a reference stream in a sense that it also describes the subsegments for any non‐indexed media stream."

Signed-off-by: Sasi Inguva <isasi@google.com>
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>