avcodec/celp_filters: Avoid invalid negation in ff_celp_lp_synthesis_filter()

Fixes: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself
Fixes: 25675/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_G729_fuzzer-4786580731199488

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

libavcodec/celp_filters.c

@@ -65,11 +65,11 @@ int ff_celp_lp_synthesis_filter(int16_t *out, const int16_t *filter_coeffs,
     int i,n;
 
     for (n = 0; n < buffer_length; n++) {
-        int sum = -rounder, sum1;
+        int sum = rounder, sum1;
         for (i = 1; i <= filter_length; i++)
-            sum += (unsigned)(filter_coeffs[i-1] * out[n-i]);
+            sum -= (unsigned)(filter_coeffs[i-1] * out[n-i]);
 
-        sum1 = ((-sum >> 12) + in[n]) >> shift;
+        sum1 = ((sum >> 12) + in[n]) >> shift;
         sum  = av_clip_int16(sum1);
 
         if (stop_on_overflow && sum != sum1)