avcodec/truespeech: Fix an integer overflow in truespeech_synth()

Fixes: signed integer overflow: 2147483188 + 2048 cannot be represented in type 'int'
Fixes: 18741/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUESPEECH_fuzzer-5748950460268544

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

libavcodec/truespeech.c

@@ -255,7 +255,7 @@ static void truespeech_synth(TSContext *dec, int16_t *out, int quart)
         int sum = 0;
         for(k = 0; k < 8; k++)
             sum += ptr0[k] * (unsigned)ptr1[k];
-        sum = out[i] + ((sum + 0x800) >> 12);
+        sum = out[i] + ((int)(sum + 0x800U) >> 12);
         out[i] = av_clip(sum, -0x7FFE, 0x7FFE);
         for(k = 7; k > 0; k--)
             ptr0[k] = ptr0[k - 1];