osdWindow: Fix setting a zero-level in osdWindow

Commit 7101cc3170bf727aebf918f37debff94ae705c85 caused a small
regression insomuch that it checks for a valid "level" but simply using
"if (level)" which will be false if level is undefined and if
level == 0.

Check for not undefined instead.

https://bugzilla.gnome.org/show_bug.cgi?id=727384