menubar: Fix hide()

When hide() was called without any calls to shown() before, an error
would occur.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/menubar/init.lua

@@ -451,7 +451,9 @@ end
 
 --- Hide the menubar.
 function menubar.hide()
+    if instance then
         instance.wibox.visible = false
+    end
 end
 
 --- Get a menubar wibox.