avcodec/apedec: make left/right unsigned to avoid undefined behavior

Fixes: signed integer overflow: 755176387 + 1515360583 cannot be represented in type 'int'
Fixes: 15506/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APE_fuzzer-5706859232624640

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 bf778af149)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/apedec.c

@@ -1376,7 +1376,7 @@ static void ape_unpack_mono(APEContext *ctx, int count)
 
 static void ape_unpack_stereo(APEContext *ctx, int count)
 {
-    int32_t left, right;
+    unsigned left, right;
     int32_t *decoded0 = ctx->decoded[0];
     int32_t *decoded1 = ctx->decoded[1];
 
@@ -1393,7 +1393,7 @@ static void ape_unpack_stereo(APEContext *ctx, int count)
 
     /* Decorrelate and scale to output depth */
     while (count--) {
-        left = *decoded1 - (*decoded0 / 2);
+        left = *decoded1 - (unsigned)(*decoded0 / 2);
         right = left + *decoded0;
 
         *(decoded0++) = left;