Fixed heap-use-after-free in await frame.

The bug was introduced in 92d10cd761e2 (0.7.0).
diff --git a/src/njs_async.c b/src/njs_async.c
index b5f6c3a..6a93cfd 100644
--- a/src/njs_async.c
+++ b/src/njs_async.c
@@ -72,6 +72,8 @@
     }
 
     async = ctx->await;
+    async->previous = vm->top_frame;
+
     function = async->function;
 
     cur_local = vm->levels[NJS_LEVEL_LOCAL];