avcodec/truespeech: Fix integer overflow in truespeech_synth()

Fixes: signed integer overflow: -1801695444 + -830224908 cannot be represented in type 'int'
Fixes: 17995/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUESPEECH_fuzzer-5648084880588800

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

@@ -254,7 +254,7 @@ static void truespeech_synth(TSContext *dec, int16_t *out, int quart)
     for(i = 0; i < 60; i++){
         int sum = 0;
         for(k = 0; k < 8; k++)
-            sum += ptr0[k] * ptr1[k];
+            sum += ptr0[k] * (unsigned)ptr1[k];
         sum = out[i] + ((sum + 0x800) >> 12);
         out[i] = av_clip(sum, -0x7FFE, 0x7FFE);
         for(k = 7; k > 0; k--)