Fix iris.util.rolling_window for degenerate masks.

Creating a mask array `data` without a mask results in `data.mask` being a single numpy boolean. This breaks the assumption in `iris.util.rolling_window()` that the mask has the same shape as the data resulting in an `IndexError`.

This PR uses `ma.getmaskarray()` to ensure that a genuine mask array (with matching shape) is used instead.