patch 8.2.2693: Vim9: locked script variable can be changed

Problem:    Vim9: locked script variable can be changed.
Solution:   Check legacy script variable for being locked. (issue #8031)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 69254be..81041bb 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1942,6 +1942,9 @@
 			store_var(name, STACK_TV_BOT(0));
 		    else
 		    {
+			SOURCING_LNUM = iptr->isn_lnum;
+			if (var_check_permission(di, name) == FAIL)
+			    goto on_error;
 			clear_tv(&di->di_tv);
 			di->di_tv = *STACK_TV_BOT(0);
 		    }