avcodec/aacdec_fixed: Limit index in vector_pow43()

Fixes: out of array access
Fixes: 26087/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-5724825462767616

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

libavcodec/aacdec_fixed.c

@@ -155,9 +155,9 @@ static void vector_pow43(int *coefs, int len)
     for (i=0; i<len; i++) {
         coef = coefs[i];
         if (coef < 0)
-            coef = -(int)ff_cbrt_tab_fixed[-coef];
+            coef = -(int)ff_cbrt_tab_fixed[(-coef) & 8191];
         else
-            coef = (int)ff_cbrt_tab_fixed[coef];
+            coef =  (int)ff_cbrt_tab_fixed[  coef  & 8191];
         coefs[i] = coef;
     }
 }