avcodec/wavarc: Fix integer overflwo in do_stereo()

Fixes: signed integer overflow: 148676193 - -2006512262 cannot be represented in type 'int'
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WAVARC_fuzzer-5963163952349184

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

libavcodec/wavarc.c

@@ -154,11 +154,11 @@ static void do_stereo(WavArcContext *s, int ch, int correlated, int len)
     } else {
         if (correlated) {
             for (int n = 0; n < nb_samples; n++)
-                s->samples[1][n + len] += s->samples[0][n + len];
+                s->samples[1][n + len] += (unsigned)s->samples[0][n + len];
         }
         for (int n = 0; n < len; n++) {
             s->pred[0][n] = s->samples[1][nb_samples + n];
-            s->pred[1][n] = s->pred[0][n] - s->samples[0][nb_samples + n];
+            s->pred[1][n] = s->pred[0][n] - (unsigned)s->samples[0][nb_samples + n];
         }
     }
 }