avcodec/ac3dec_fixed: Fix runtime error: left shift of 419 by 23 places cannot be represented in type 'int'

Fixes: 1352/clusterfuzz-testcase-minimized-5757565017260032

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

libavcodec/ac3dec_fixed.c

@@ -69,7 +69,7 @@ static void scale_coefs (
     int temp, temp1, temp2, temp3, temp4, temp5, temp6, temp7;
 
     mul = (dynrng & 0x1f) + 0x20;
-    shift = 4 - ((dynrng << 23) >> 28);
+    shift = 4 - (sign_extend(dynrng, 9) >> 5);
     if (shift > 0 ) {
       round = 1 << (shift-1);
       for (i=0; i<len; i+=8) {