patch 8.2.3511: Vim9: entry for loop variable is created every round

Problem:    Vim9: entry for loop variable is created every round.
Solution:   Only create the entry once. (closes #8996)
diff --git a/src/vim9script.c b/src/vim9script.c
index ca245b2..9696dfe 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -978,8 +978,9 @@
 	// legacy script doesn't store variable types
 	return NULL;
 
-    // Find the svar_T in sn_var_vals.
-    for (idx = 0; idx < si->sn_var_vals.ga_len; ++idx)
+    // Find the svar_T in sn_var_vals.  Start at the end, in a for loop the
+    // variable was added at the end.
+    for (idx = si->sn_var_vals.ga_len - 1; idx >= 0; --idx)
     {
 	svar_T    *sv = ((svar_T *)si->sn_var_vals.ga_data) + idx;