Prevent panic in Notebook.PrependPage.
Prevent panic in Notebook.PrependPage.

It is valid for a notebook's page's tab label to be NULL.  This change
prevents a nil pointer dereference when passing a nil tab label to the
PrependPage method by explicitly checking for a nil interface value.

andre-hub/gotk3,visionect/gotk3,pekim/gotk3,LittleBuster/gotk3,pekim/gotk3,visionect/gotk3,andre-hub/gotk3,conformal/gotk3,gotk3/gotk3,yamnikov-oleg/gotk3,yamnikov-oleg/gotk3,Francesco149/gotk3,yamnikov-oleg/gotk3,andre-hub/gotk3,LittleBuster/gotk3,conformal/gotk3,gotk3/gotk3,Francesco149/gotk3,pekim/gotk3