avformat/flvdec: initialize context before reading from it

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f9b8387069e_5377_flv_with_pcm_s16be_audio_track.flv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
