unlimited scrolling on pianoroll caused fpe

[ferprojo]

Found that when I accidentally scrolled too much on the piano roll window, it triggered a floating point exception. I looked at how the Editor implemented set_samples_per_pixel, and did the same, just returning when spp < 1. I'm new to ardour development so I don't know if this is correct, but it fixed it for me.

[x42]

I'd assume the better solution would be if (spp < 1) { spp = 1; }
Then again ideally that case should not happen in the first place, and we just add an assert (spp > 0);