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/GTabWidget.cpp

@@ -45,13 +45,18 @@ void GTabWidget::resize_event(GResizeEvent& event)
 
 Rect GTabWidget::child_rect_for_size(const Size& size) const
 {
+    Rect rect;
     switch (m_tab_position) {
     case TabPosition::Top:
-        return { { container_padding(), bar_height() + container_padding() }, { size.width() - container_padding() * 2, size.height() - bar_height() - container_padding() * 2 } };
+        rect = { { container_padding(), bar_height() + container_padding() }, { size.width() - container_padding() * 2, size.height() - bar_height() - container_padding() * 2 } };
+        break;
     case TabPosition::Bottom:
-        return { { container_padding(), container_padding() }, { size.width() - container_padding() * 2, size.height() - bar_height() - container_padding() * 2 } };
+        rect = { { container_padding(), container_padding() }, { size.width() - container_padding() * 2, size.height() - bar_height() - container_padding() * 2 } };
+        break;
     }
-    ASSERT_NOT_REACHED();
+    if (rect.is_empty())
+        return {};
+    return rect;
 }
 
 void GTabWidget::child_event(CChildEvent& event)

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;