Offset applied to longslit edges causes crash for 46-slit longslits

[mheida]

I ran into this issue while reducing data taken with a full 46 unit longslit mask. AutoDriver produces a description of the longslit with an upper and lower boundary, which includes a 10 pixel offset to the calculated values. For the 46 unit slit that gives boundaries of [10, 2058], and that causes the make_pixel_flat to crash with an index error because the maximum index is 2048. I suppose this is really only an issue for the 46 unit longslit, and it's an easy manual fix, but it might be worth capping that upper boundary at 2048 in AutoDriver or something.