avformat/mp3: large id3 tags break concatenated file detection

If the file size is much larger than what is indicated in the XING
header, the demuxer assumes it's a concatenated file, and throws away
the (presumably) incorrect duration information. Unfortunately, this
also triggers if the id3 tags are very large (embedded pictures and
such). Then the half-baked heuristic not only breaks the duration
display, but also gapless audio.

Fix it by subtracting the size of the headers (the check is off by some
bytes, but that doesn't matter at all). Note that there could be an
arbitrary amount of tags _after_ the mp3 data, but hopefully these are
not too large to trigger the heuristic in practice.

Also add a warning when this happens.

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