core: Avoid ClassCastException in getExtension(Class)

Ensure that he returned extension element is actually of the correct
type. It should not be common, but in theory, Smack's provider
architecture would allow for different types to be returned than the
ones one may expect.

smack-core/src/main/java/org/jivesoftware/smack/packet/StanzaView.java

@@ -87,10 +87,14 @@ public interface StanzaView extends XmlLangElement {
         return false;
     }
 
-    @SuppressWarnings("unchecked")
     default <E extends ExtensionElement> E getExtension(Class<E> extensionElementClass) {
         QName qname = XmppElementUtil.getQNameFor(extensionElementClass);
-        return (E) getExtension(qname);
+        ExtensionElement extensionElement = getExtension(qname);
+        if (!extensionElementClass.isInstance(extensionElement)) {
+            return null;
+        }
+
+        return extensionElementClass.cast(extensionElement);
     }
 
     /**