patch 8.2.2703: Vim9: memory leak when failing on locked variable

Problem:    Vim9: memory leak when failing on locked variable.
Solution:   Free the memory.
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 867afaf..0b1d5fc 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -1323,6 +1323,7 @@
   unlet g:var_test
   unlet g:var_prefixed
   unlet g:other_var
+  unlet g:globConst
   unlet g:FOO
   unlet g:FOOS
   unlet g:FLIST
@@ -1375,7 +1376,7 @@
     SetGlobalConst()
     g:globConst = 234
   END
-  CheckScriptFailure(lines, 'E741: Value is locked: globConst', 1)
+  CheckScriptFailure(lines, 'E741: Value is locked: g:globConst', 6)
   unlet g:globConst
 
   lines =<< trim END