avcodec/alsdec: Fix integer overflow of raw_samples in decode_blocks()

Fixes: signed integer overflow: 2147483424 - -1772303236 cannot be represented in type 'int'
Fixes: 15708/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5067890362941440

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

libavcodec/alsdec.c

@@ -1175,10 +1175,10 @@ static int decode_blocks(ALSDecContext *ctx, unsigned int ra_frame,
                 av_log(ctx->avctx, AV_LOG_WARNING, "Invalid channel pair.\n");
 
             for (s = 0; s < div_blocks[b]; s++)
-                bd[0].raw_samples[s] = bd[1].raw_samples[s] - bd[0].raw_samples[s];
+                bd[0].raw_samples[s] = bd[1].raw_samples[s] - (unsigned)bd[0].raw_samples[s];
         } else if (bd[1].js_blocks) {
             for (s = 0; s < div_blocks[b]; s++)
-                bd[1].raw_samples[s] = bd[1].raw_samples[s] + bd[0].raw_samples[s];
+                bd[1].raw_samples[s] = bd[1].raw_samples[s] + (unsigned)bd[0].raw_samples[s];
         }
 
         offset  += div_blocks[b];