avcodec/h264: clear dequant8_coeff pointers if 8x8 mode is not enabled

This prevents stale pointers from being left
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -1314,6 +1314,8 @@ static void init_dequant_tables(H264Context *h)
 {
     int i, x;
     init_dequant4_coeff_table(h);
+    memset(h->dequant8_coeff, 0, sizeof(h->dequant8_coeff));
+
     if (h->pps.transform_8x8_mode)
         init_dequant8_coeff_table(h);
     if (h->sps.transform_bypass) {