avformat/mov: prevent duplication of first fragment's ctts_data

MP4 files with fragments might have the first moof box that is mentioned
in a fragment index before the first mdat box. Since it is then already
parsed by mov_read_header, we have to make sure that mov_switch_root
will not parse it again when seeking by setting the headers_read flag in
the index. Parsing it a second time would cause the ctts_data array to
receive a second copy of the information from the trun box, leading to
wrong PTS values for the second and following fragments in presence of
B-frames.

Fixes ticket 6560.

Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
