avutil/fixed_dsp: Fix integer overflows in butterflies_fixed_c()

Fixes: signed integer overflow: 0 - -2147483648 cannot be represented in type 'int'
Fixes: 23646/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-5480991098667008

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

libavutil/fixed_dsp.c

@@ -134,9 +134,10 @@ static int scalarproduct_fixed_c(const int *v1, const int *v2, int len)
     return (int)(p >> 31);
 }
 
-static void butterflies_fixed_c(int *v1, int *v2, int len)
+static void butterflies_fixed_c(int *v1s, int *v2, int len)
 {
     int i;
+    unsigned int *v1 = v1s;
 
     for (i = 0; i < len; i++){
         int t = v1[i] - v2[i];