Randomize non-ACGT and then build colors by --load-dbg may fail?

[jnalanko]

When using the randomization for non-ACGT characters without building colors, and building colors again on another run, the program should crash because if we don't get exactly the same random choices, some of the k-mers will change and this messes up the coloring. But at the moment it works because we don't give a seed value for std::rand so according to the standard it will always be seeded with the value 1. No other piece of code uses std::rand before the coloring so by luck we happen to get the random choices. But this might break in the future, so it should be fixed.