avcodec/vp8: Fix null pointer dereference in ff_vp8_decode_free()

Fixes: signal_sigsegv_d5de23_967_vp80_00_comprehensive_010.ivf with memlimit 524288

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