avcodec/ac3dec_fixed: Fix integer overflow in scale_coefs()

Fixes: runtime error: signed integer overflow: 2147483520 + 128 cannot be represented in type 'int'
Fixes: 4800/clusterfuzz-testcase-minimized-6110372403609600

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a1f38c7589)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3dec_fixed.c

@@ -64,8 +64,8 @@ static void scale_coefs (
     int dynrng,
     int len)
 {
-    int i, shift, round;
-    unsigned mul;
+    int i, shift;
+    unsigned mul, round;
     int temp, temp1, temp2, temp3, temp4, temp5, temp6, temp7;
 
     mul = (dynrng & 0x1f) + 0x20;