patch 8.2.2461: Coverity warns for unchecked return value

Problem:    Coverity warns for unchecked return value.
Solution:   Add "(void)" to avoid the warning.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 8a5708c..8728088 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -897,11 +897,11 @@
     {
 	case VAR_LIST:
 	    if (tv->vval.v_list == NULL)
-		rettv_list_alloc(tv);
+		(void)rettv_list_alloc(tv);
 	    break;
 	case VAR_DICT:
 	    if (tv->vval.v_dict == NULL)
-		rettv_dict_alloc(tv);
+		(void)rettv_dict_alloc(tv);
 	    break;
 	default:
 	    break;