avformat/mp3enc: Assert that the header we assembled is valid

Silences: CID1351343

The header is calculated by the code above the changed hunk, it is
thus asserted that the header is always correct.

Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
