avcodec/alsdec: fix undefined shift in multiply()

Fixes: left shift of negative value -6
Fixes: 15564/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5701655938465792

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

libavcodec/alsdec.c

@@ -1416,7 +1416,7 @@ static SoftFloat_IEEE754 multiply(SoftFloat_IEEE754 a, SoftFloat_IEEE754 b) {
         return_val = 0x80000000U;
     }
 
-    return_val |= (a.exp + b.exp + bit_count - 47) << 23;
+    return_val |= ((unsigned)av_clip(a.exp + b.exp + bit_count - 47, -126, 127) << 23) & 0x7F800000;
     return_val |= mantissa;
     return av_bits2sf_ieee754(return_val);
 }