unified2: fix memory leak at shutdown

Module didn't properly free output context at shutdown. Led to a leak
in Unix Socket mode.

src/alert-unified2-alert.c

@@ -1465,6 +1465,7 @@ static void Unified2AlertDeInitCtx(OutputCtx *output_ctx)
             }
             SCFree(unified2alert_ctx);
         }
+        SCFree(output_ctx);
     }
 }