patch 8.2.4771: Coverity warns for not checking return value

Problem:    Coverity warns for not checking return value.
Solution:   Check return value of rettv_dict_alloc().
diff --git a/src/channel.c b/src/channel.c
index ef747d0..79f4dbc 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4549,7 +4549,8 @@
 	dictitem_T	*di;
 
 	// return an empty dict by default
-	rettv_dict_alloc(rettv);
+	if (rettv_dict_alloc(rettv) == FAIL)
+	    return;
 
 	if (argvars[1].v_type != VAR_DICT)
 	{