avcodec/shorten: Fix integer overflow with offset

Fixes: signed integer overflow: -1625810908 - 582229060 cannot be represented in type 'int'
Fixes: 10977/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5732602018267136

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/shorten.c

@@ -382,7 +382,7 @@ static int decode_subframe_lpc(ShortenContext *s, int command, int channel,
     /* subtract offset from previous samples to use in prediction */
     if (command == FN_QLPC && coffset)
         for (i = -pred_order; i < 0; i++)
-            s->decoded[channel][i] -= coffset;
+            s->decoded[channel][i] -= (unsigned)coffset;
 
     /* decode residual and do LPC prediction */
     init_sum = pred_order ? (command == FN_QLPC ? s->lpcqoffset : 0) : coffset;
@@ -397,7 +397,7 @@ static int decode_subframe_lpc(ShortenContext *s, int command, int channel,
     /* add offset to current samples */
     if (command == FN_QLPC && coffset)
         for (i = 0; i < s->blocksize; i++)
-            s->decoded[channel][i] += coffset;
+            s->decoded[channel][i] += (unsigned)coffset;
 
     return 0;
 }