focus: fall back to old behaviour if we didn't focus anything

focus: fall back to old behaviour if we didn't focus anything

8455b9ac74 seems to have introduced a problem where we can wind
up focusing no widget at all if the `while (parent)` loop doesn't
find a widget it can successfully move the focus to. This 'fixes'
that by falling back to doing the previous thing if we make it
all the way through that loop without moving the focus. Thanks to
@coreyberla for a hint to improve the implementation.

Signed-off-by: Adam Williamson <awilliam@redhat.com>