fix warning about possibly uninitialized variable in release build

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59270 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/scrolwin.cpp

@@ -164,6 +164,10 @@ GtkPolicyType GtkPolicyFromWX(wxScrollbarVisibility visibility)
             policy = GTK_POLICY_AUTOMATIC;
             break;
 
+        default:
+            wxFAIL_MSG( wxS("unknown scrollbar visibility") );
+            // fall through
+
         case wxSHOW_SB_ALWAYS:
             policy = GTK_POLICY_ALWAYS;
             break;