avcodec/ac3dec_fixed: Fix multiple runtime error: signed integer overflow: -39271008 * 59 cannot be represented in type 'int'

Fixes: 2113/clusterfuzz-testcase-minimized-6510704959946752

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 4e3ab1a5c1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3dec_fixed.c

@@ -65,7 +65,7 @@ static void scale_coefs (
     int len)
 {
     int i, shift, round;
-    int16_t mul;
+    unsigned mul;
     int temp, temp1, temp2, temp3, temp4, temp5, temp6, temp7;
 
     mul = (dynrng & 0x1f) + 0x20;