avcodec/wavarc: fix signed integer overflow in block type 6/19

Fixes: signed integer overflow: -2088796289 + -91276551 cannot be represented in type 'int'
Fixes: 67772/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WAVARC_fuzzer-6533568953122816

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

@@ -647,7 +647,7 @@ static int decode_5elp(AVCodecContext *avctx,
                 for (int o = 0; o < order; o++)
                     sum += s->filter[ch][o] * (unsigned)samples[n + 70 - o - 1];
 
-                samples[n + 70] += ac_out[n] + (sum >> 4);
+                samples[n + 70] += ac_out[n] + (unsigned)(sum >> 4);
             }
 
             for (int n = 0; n < 70; n++)