Fix ribbon panel size after calling Realize() again

Reset m_smallest_unminimised_size variable in Realize() before calling
GetPanelSizerMinSize() to prevent the latter from using the old value of
that variable, which could result in panel elements being cut off.

Closes #18226.

src/ribbon/panel.cpp

@@ -671,6 +671,10 @@ bool wxRibbonPanel::Realize()
 
     wxSize minimum_children_size(0, 0);
 
+    // Reset it before calling GetPanelSizerMinSize() below as it shouldn't use
+    // the old value, if we had any.
+    m_smallest_unminimised_size = wxDefaultSize;
+
     // Ask sizer if there is one present
     if(GetSizer())
     {