emotion: Check engine for NULL before dereferencing.

It might be that our engine list is empty and thus our engine might still
be set to NULL. Better check and return if this is the case instead of
dereferencing NULL.

src/modules/emotion/gstreamer/emotion_sink.c

@@ -1027,6 +1027,8 @@ _image_resize(void *data, Evas *e EINA_UNUSED, Evas_Object *obj, void *event_inf
                }
           }
 
+        if (!engine) return;
+
         if (strstr(engine, "software") != NULL)
           {
              caps = gst_caps_new_simple("video/x-raw-rgb",