LibGUI: GTabWidget should not set children to have negative size

This could happen if a child was added to a GTabWidget before the
GTabWidget had its first layout.

Also add an assertion to catch this in GWidget::set_relative_rect()
since it was not immediately obvious what was happening.

Libraries/LibGUI/GWidget.cpp

@@ -44,6 +44,8 @@ void GWidget::child_event(CChildEvent& event)
 
 void GWidget::set_relative_rect(const Rect& rect)
 {
+    ASSERT(rect.width() >= 0 && rect.height() >= 0);
+
     if (rect == m_relative_rect)
         return;