BUG: Error in rolling_var if window is larger than array, fixes #7297
fixes #7297

Other rolling window functions rely on the logic in `_check_minp`, and split the iteration into the ranges `[0, minp-1)` and `(minp-1, N)`. Because of the way `rolling_var` handles things, splitting the iteration into `[0, win)` and `(win, N)` makes more sense. Added also some comments as to what is going on elsewhere in the code.

No tests have been added, as @kdiether seems to have that very advanced.

@jaimefrio thanks for the fix, just need tests from #7297 (If you want to add ok too). I don't think cc @kdiether has pull his request (yet)
 @jaimefrio can you incorporate those tests in the #7297 (if you want to add more complete tests go for it as discussed there). but want to put this in place for 0.14.1
 Sure, hopefully will get it done tonight, tomorrow at the latest. Will ping here once the tests are in and Travis is OKing it.
 pls add a release note in v0.14.1.txt as well thxs
 hows this coming?
 Let's see what Travis thinks, but I think this should be it.
 The error is unrelated, some Travis setup failure.
 @jaimefrio thanks for the fixes! (sometimes travis get stuck....I just restarted and all good)
