Make image data contiguous before passing it to libmagickwand.

This fixes #97

and also a typo, I didn't squash because it's two different things.

By the way, note that [the line computing `n`](https://github.com/timholy/Images.jl/blob/4a98/src%2Fio.jl#L253) is unnecessary, as `n` is unused. I guess you wanted to use this to write a stack of slices of a 3D image/video into the wand but never got around to it? Thus I left it in there.