patch 8.2.2692: Vim9: locked script variable can be changed

Problem:    Vim9: locked script variable can be changed.
Solution:   Check for locked value. (closes #8031)

src/testdir/test_vim9_assign.vim

@@ -1346,6 +1346,17 @@ def Test_var_declaration_fails()
   CheckScriptFailure(lines, 'E741:')
   unlet g:constvar
 
+  lines =<< trim END
+    vim9script
+    var name = 'one'
+    lockvar name
+    def SetLocked()
+      name = 'two'
+    enddef
+    SetLocked()
+  END
+  CheckScriptFailure(lines, 'E741: Value is locked: name')
+
   lines =<< trim END
     vim9script
     def SetGlobalConst()