avcodec/aacdec_fixed: Fix undefined shift

Fixes: runtime error: left shift of negative value -801112064
Fixes: 3492/clusterfuzz-testcase-minimized-5784775283441664

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

libavcodec/aacdec_fixed.c

@@ -309,7 +309,7 @@ static av_always_inline void predict(PredictorState *ps, int *coef,
             if (shift > 0) {
                 *coef += (unsigned)((pv.mant + (1 << (shift - 1))) >> shift);
             } else
-                *coef += (unsigned)(pv.mant << -shift);
+                *coef += (unsigned)pv.mant << -shift;
         }
     }