patch 8.2.3649: Vim9: error for variable declared in while loop

Problem:    Vim9: error for variable declared in while loop.
Solution:   Do not keep the first variable. (closes #9191)
diff --git a/src/ex_eval.c b/src/ex_eval.c
index d80500e..4e38e96 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -1201,9 +1201,10 @@
 								& CSF_FUNC_DEF;
 
 		// Any variables defined in the previous round are no longer
-		// visible.  Keep the first one, it is the loop variable that
-		// we reuse every time around.
-		for (i = cstack->cs_script_var_len[cstack->cs_idx] + 1;
+		// visible.  Keep the first one for ":for", it is the loop
+		// variable that we reuse every time around.
+		for (i = cstack->cs_script_var_len[cstack->cs_idx]
+					  + (eap->cmdidx == CMD_while ? 0 : 1);
 					       i < si->sn_var_vals.ga_len; ++i)
 		{
 		    svar_T	*sv = ((svar_T *)si->sn_var_vals.ga_data) + i;