Speedups for unused Huffman groups.

If we know a Huffman group is unused, do not build
it: just check that it is valid.

Change-Id: Ie8223fe1afa1e769085a23ab92e730edd9060176

src/utils/huffman_utils.h

@@ -78,6 +78,8 @@ void VP8LHtreeGroupsFree(HTreeGroup* const htree_groups);
 // the huffman table.
 // Returns built table size or 0 in case of error (invalid tree or
 // memory error).
+// If root_table is NULL, it returns 0 if a lookup cannot be built, something
+// > 0 otherwise (but not the table size).
 int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
                           const int code_lengths[], int code_lengths_size);