Fix for #69: problems caused by uninitialized pointer to mutex

Created by: aroffringa

As reported by David, this can result in weird behaviour including a segfault.

I've also removed the default empty constructor of Predict(), as it was used to create an empty Predict class that would later be assigned too. However, this is a bit awkward with such a complex class -- better to make it nullable by putting it inside a unique_ptr.