avutil/tx: avoid negative left shifts

Fixes: left shift of negative value -1
Fixes: 33736/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SIREN_fuzzer-6657785795313664

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

libavutil/tx.c

@@ -97,9 +97,9 @@ static inline int split_radix_permutation(int i, int m, int inverse)
     if (m <= 1)
         return i & 1;
     if (!(i & m))
-        return (split_radix_permutation(i, m, inverse) << 1);
+        return split_radix_permutation(i, m, inverse) * 2;
     m >>= 1;
-    return (split_radix_permutation(i, m, inverse) << 2) + 1 - 2*(!(i & m) ^ inverse);
+    return split_radix_permutation(i, m, inverse) * 4 + 1 - 2*(!(i & m) ^ inverse);
 }
 
 int ff_tx_gen_ptwo_revtab(AVTXContext *s, int invert_lookup)