avcodec/ffv1dec: Clear quant_table_count if its invalid

Fixes deallocation of corrupted pointer
Fixes: 343dfbe142a38b521ed069dc4ea7c03b/signal_sigsegv_421427_4074_ffb11959610278cd40dbc153464aa254.avi
No releases affected

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