avcodec/alsdec: fix mantisse shift

Fixes: shift exponent -1 is negative
Fixes: 16039/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5656825657032704

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

libavcodec/alsdec.c

@@ -1404,7 +1404,11 @@ static SoftFloat_IEEE754 multiply(SoftFloat_IEEE754 a, SoftFloat_IEEE754 b) {
         }
     }
 
-    mantissa = (unsigned int)(mantissa_temp >> cutoff_bit_count);
+    if (cutoff_bit_count >= 0) {
+        mantissa = (unsigned int)(mantissa_temp >> cutoff_bit_count);
+    } else {
+        mantissa = (unsigned int)(mantissa_temp <<-cutoff_bit_count);
+    }
 
     // Need one more shift?
     if (mantissa & 0x01000000ul) {