Fix error-list jumping when no error on line.

Take a look at video showing default behavior as flycheck 0.21 snapshot. Error list is jumping when moving cursor between lines with and without error.
https://www.youtube.com/watch?v=PyN6huUR-jc&feature=youtu.be


And this is behavior with my small patch:
https://www.youtube.com/watch?v=EeXYnP13P6c&feature=youtu.be

Recenter is still functional, but IMHO error-list is less annoying.
Don't nest conditionals please.  Join this line and the `unless` together.