avformat/aiffenc: Don't forget chapters

avformat/aiffenc: Don't forget chapters

If the write_id3v2 option is set, the aiff muxer would write id3v2 tags
if there is global metadata or if there are attached pics to write.
Chapters are ignored in this check that precedes writing id3v2 tags.
Yet 47ac344970f1d6ef149c4b8a883b68cdb15112c2 added support for writing
chapters as id3v2 tags, so one should check for the existence of chapters,
too; otherwise the chapters would only be written in case there is
global metadata or an attached pic.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>