patch 9.0.1569: cannot use "this.member" in lambda in class method

Problem:    Cannot use "this.member" in lambda in class method.
Solution:   Adjust check for reserved keyword. (Hirohito Higashi,
            closes #12416, closes #12076, closes #12336)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 265f846..e666357 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1580,7 +1580,8 @@
 	else
 	{
 	    // No specific kind of variable recognized, just a name.
-	    if (check_reserved_name(lhs->lhs_name, cctx) == FAIL)
+	    if (check_reserved_name(lhs->lhs_name, lhs->lhs_has_index
+						&& *var_end == '.') == FAIL)
 		return FAIL;
 
 	    if (lookup_local(var_start, lhs->lhs_varlen,