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>
(cherry picked from commit e0412607)

Fixes ticket #5052.

libavcodec/ffv1dec.c

@@ -556,6 +556,7 @@ static int read_extra_header(FFV1Context *f)
     f->quant_table_count = get_symbol(c, state, 0);
     if (f->quant_table_count > (unsigned)MAX_QUANT_TABLES || !f->quant_table_count) {
         av_log(f->avctx, AV_LOG_ERROR, "quant table count %d is invalid\n", f->quant_table_count);
+        f->quant_table_count = 0;
         return AVERROR_INVALIDDATA;
     }