avcodec/shorten: Fix multiple integer overflows

Fixes: signed integer overflow: 3 * 1006632960 cannot be represented in type 'int'
Fixes: 8278/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5692857166856192

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 f2abd36b38)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -313,7 +313,7 @@ static int decode_subframe_lpc(ShortenContext *s, int command, int channel,
     for (i = 0; i < s->blocksize; i++) {
         sum = init_sum;
         for (j = 0; j < pred_order; j++)
-            sum += coeffs[j] * s->decoded[channel][i - j - 1];
+            sum += coeffs[j] * (unsigned)s->decoded[channel][i - j - 1];
         s->decoded[channel][i] = get_sr_golomb_shorten(&s->gb, residual_size) +
                                  (sum >> qshift);
     }
@@ -593,7 +593,7 @@ static int shorten_decode_frame(AVCodecContext *avctx, void *data,
 
             /* update means with info from the current block */
             if (s->nmean > 0) {
-                int32_t sum = (s->version < 2) ? 0 : s->blocksize / 2;
+                int64_t sum = (s->version < 2) ? 0 : s->blocksize / 2;
                 for (i = 0; i < s->blocksize; i++)
                     sum += s->decoded[channel][i];