avcodec/pcm-dvd: reset last header on errors

Fixes: msan_uninit-mem_7f4fff975a2c_4957_dvd_audio_sample.aob
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>