avcodec/audiodsp: Fix integer overflow in scalarproduct_int16_c()

Fixes: signed integer overflow: 2145417478 + 76702564 cannot be represented in type 'int'
Fixes: 20313/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RA_144_fuzzer-5734487724130304

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

libavcodec/audiodsp.c

@@ -79,7 +79,7 @@ static void vector_clipf_c(float *dst, const float *src, int len,
 static int32_t scalarproduct_int16_c(const int16_t *v1, const int16_t *v2,
                                      int order)
 {
-    int res = 0;
+    unsigned res = 0;
 
     while (order--)
         res += *v1++ **v2++;