trayicon-x11: Fix warnings when visual is not available.
trayicon-x11: Fix warnings when visual is not available.

This fixes a regression introduced in commit
b1a1685a33eca16979e63d79915395af0b36. gdk_visual_get_*_pixel_details()
should not be called with a NULL visual.

https://bugzilla.gnome.org/show_bug.cgi?id=649588

Unity-Technologies/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Unity-Technologies/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk