cngdec: Fix the memset size to cover the full array

This was a leftover from previous iterations of the code, where the
refl coef arrays were statically allocated.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/cngdec.c

@@ -108,7 +108,7 @@ static int cng_decode_frame(AVCodecContext *avctx, void *data,
     if (avpkt->size) {
         float dbov = -avpkt->data[0] / 10.0;
         p->target_energy = 1081109975 * pow(10, dbov) * 0.75;
-        memset(p->target_refl_coef, 0, sizeof(p->refl_coef));
+        memset(p->target_refl_coef, 0, p->order * sizeof(*p->target_refl_coef));
         for (i = 0; i < FFMIN(avpkt->size - 1, p->order); i++) {
             p->target_refl_coef[i] = (avpkt->data[1 + i] - 127) / 128.0;
         }