avcodec/flashsv: Check size before updating it

Fixes out of array read
Fixes: 3c857d4d90365731524716e6d051e43a/signal_sigsegv_7f4f59bcc29e_1386_20abd2c8e655cb9c75b24368e65fe3b1.flv

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