fix memory leak in DocumentObject::execute and return ASAP on errors

src/App/DocumentObject.cpp

@@ -82,8 +82,11 @@ DocumentObjectExecReturn *DocumentObject::execute(void)
 {
     //call all extensions
     auto vector = getExtensionsDerivedFromType<App::DocumentObjectExtension>();
-    for(auto ext : vector)
-        ext->extensionExecute();
+    for(auto ext : vector) {
+        auto ret = ext->extensionExecute();
+        if (ret != StdReturn)
+            return ret;
+    }
 
     return StdReturn;
 }