patch 8.2.4682: Vim9: can use :unlockvar for const variable

Problem:    Vim9: can use :unlockvar for const variable. (Ernie Rael)
Solution:   Check whether the variable is a const.
diff --git a/src/eval.c b/src/eval.c
index 2cde642..b30577f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1065,7 +1065,7 @@
 		&& lp->ll_tv == &v->di_tv
 		&& ht != NULL && ht == get_script_local_ht())
 	{
-	    svar_T  *sv = find_typval_in_script(lp->ll_tv, 0);
+	    svar_T  *sv = find_typval_in_script(lp->ll_tv, 0, TRUE);
 
 	    // Vim9 script local variable: get the type
 	    if (sv != NULL)