avcodec/sanm: Reset sizes in destroy_buffers()

Fixes crash in 1288a2fe8e9ae6b00ca40e089d08ca65_signal_sigsegv_7ffff71426a7_354_accident.san with allocation limit 65536

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
