Fix for scalar solves crashes in Rapthor because wrong npol

Somehow this error was made in !444 (merged), commit d05a3bbd. Apparently I caught in my own review that I had set nPol incorrectly to 1 in the diagonal solver, and then I made it worse by changing it at the wrong position (in the scalar solver) to 2.

Even though the error was made back in May, this only recently became an issue in Rapthor, because only recently the solver formally known as "Bda" solver is used in all cases (basically since MR !569 (merged) ).