Avoid using quickfix window

If I run `:Qdo` while I'm in the quickfix window, one of the processed files ends up in the quickfix window, overriding its contents. A `copen` afterwards would work just fine, but this leaves a split window with an unexpected size in the bottom of the screen. The solution I came up with is to simply `:cclose` it if necessary and then `:copen` it again.