avcodec/flacdec: avoid undefined shift

Fixes: shift exponent 32 is too large for 32-bit type 'unsigned int'
Fixes: 4688/clusterfuzz-testcase-minimized-6572210748653568

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

libavcodec/flacdec.c

@@ -460,7 +460,7 @@ static inline int decode_subframe(FLACContext *s, int channel)
         return AVERROR_INVALIDDATA;
     }
 
-    if (wasted) {
+    if (wasted && wasted < 32) {
         int i;
         for (i = 0; i < s->blocksize; i++)
             decoded[i] = (unsigned)decoded[i] << wasted;