avcodec/dstdec: Fix integer overflow in read_table()

Fixes: signed integer overflow: -16 * 134217879 cannot be represented in type 'int'
Fixes: 20492/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DST_fuzzer-5639509530378240

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2d465a401d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dstdec.c

@@ -155,7 +155,7 @@ static int read_table(GetBitContext *gb, Table *t, const int8_t code_pred_coeff[
             for (j = method + 1; j < t->length[i]; j++) {
                 int c, x = 0;
                 for (k = 0; k < method + 1; k++)
-                    x += code_pred_coeff[method][k] * t->coeff[i][j - k - 1];
+                    x += code_pred_coeff[method][k] * (unsigned)t->coeff[i][j - k - 1];
                 c = get_sr_golomb_dst(gb, lsb_size);
                 if (x >= 0)
                     c -= (x + 4) / 8;