patch 8.2.1869: Vim9: memory leak when using add()

Problem:    Vim9: memory leak when using add().
Solution:   Free the added item.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 7a5871a..115212f 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2308,6 +2308,7 @@
 		    }
 		    if (list_append_tv(l, tv2) == FAIL)
 			goto failed;
+		    clear_tv(tv2);
 		    --ectx.ec_stack.ga_len;
 		}
 		break;