avformat/mp3dec: read xing toc independant of usetoc, only skip filling index if requested

Fixes hypothetical parsing bug with -usetoc 0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mp3dec: read TOC even if not all needed information is available

This fixes hyothetical bugs with parsing of elements after the TOC

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>