message: vertexcodec: Fix unindexed encoding
  Disable prediction for vertices that couldn't be compressed using pgram
  prediction or if the caller didn't specify index data in the first
  place.
diff: @@ -32,7 +32,7 @@ static unsigned char* encodeVertexBlock(unsigned char* data, const unsigned char
  {
  unsigned char p = vertex_data[vertex_offset - vertex_size];
  - if (prediction[i])
  + if (prediction && prediction[i])
  {
  unsigned char pa = prediction[i] >> 16;
  unsigned char pb = prediction[i] >> 8;
  