F449: Ignores last embedded layer

Merge request !3039 (merged) counts the number of embedded layers in use in a couple of places (decoder_decode and avm_decode_frame_from_obus):

      int num_mlayers = 0;
      ...
      for (int i = 0; i < MAX_NUM_MLAYERS - 1; i++) {
        if (pbi->mlayer_id_map[i] >= 0) num_mlayers++;
      }

MAX_NUM_MLAYERS is equal to 8, so this will check the first seven mlayers, but ignore the eighth.

This looks like a coding bug and i < MAX_NUM_MLAYERS - 1 should be changed to i < MAX_NUM_MLAYERS.

The same bug appears twice in the merge request.

Note that the counting code for extended layers also subtracts 1, but in this case it makes more sense because extended layer 31 has a special meaning. I suspect there was a copy and paste error when writing the embedded layers code.