avcodec/mpeg12dec: Fix uninitialized data in fate-sub-cc-scte20

The issue is introduced in a705bcd7, please tested with below command line:
make V=1 fate-sub-cc-scte20 TARGET_EXEC="valgrind --error-exitcode=1"
Reported-by: NMartin Storsjö <martin@martin.st>
Reviewed-by: NMartin Storsjö <martin@martin.st>
Signed-off-by: NLimin Wang <lance.lmwang@gmail.com>

libavcodec/mpeg12dec.c

@@ -2276,6 +2276,8 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
             if (ret >= 0) {
                 uint8_t field, cc1, cc2;
                 uint8_t *cap = s1->a53_buf_ref->data;
+
+                memset(s1->a53_buf_ref->data + old_size, 0, cc_count * 3);
                 for (i = 0; i < cc_count && get_bits_left(&gb) >= 26; i++) {
                     skip_bits(&gb, 2); // priority
                     field = get_bits(&gb, 2);