auenc: remove put_au_header() and merge its code into au_write_header

Such level of abstraction is pointless.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
auenc: strict check for supported codec

Also check number of streams and give error message why muxing failed.
This prevents muxing unsupported codec with known and supported tag.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
flashsv: clear blocks array on reallocation

Fixes use of uninitialized data

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>