fix memory leak in obj create functions if ext alloc fails

src/lv_objx/lv_slider.c

@@ -70,7 +70,10 @@ lv_obj_t * lv_slider_create(lv_obj_t * par, const lv_obj_t * copy)
     /*Allocate the slider type specific extended data*/
     lv_slider_ext_t * ext = lv_obj_allocate_ext_attr(new_slider, sizeof(lv_slider_ext_t));
     LV_ASSERT_MEM(ext);
-    if(ext == NULL) return NULL;
+    if(ext == NULL) {
+        lv_obj_del(new_slider);
+        return NULL;
+    }
 
     /*Initialize the allocated 'ext' */
     ext->style_knob = &lv_style_pretty;