Fix null pointer call in QMetaType::destruct().

Logic has a flow causing null pointer call during destruction of an
instance of a custom type in bootstrap mode.

The patch doesn't include autotests, because we do not test bootstrap
mode.

Change-Id: Ie70c6f4226cb604f094513621d96a560e84502d7
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>

src/corelib/kernel/qmetatype.cpp

@@ -1546,8 +1546,10 @@ class TypeDestructor {
                 if (!qMetaTypeWidgetsHelper)
                     return;
                 dtor = qMetaTypeWidgetsHelper[type - QMetaType::FirstWidgetsType].destructor;
-            } else
+            } else {
                 customTypeDestructor(type, where);
+                return;
+            }
             dtor(where);
         }
     };