Fix elif start column offset when there is an else following (GH-17596)

diff --git a/Python/ast.c b/Python/ast.c
index c450b87..55fe58c 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -4050,8 +4050,8 @@
 
             asdl_seq_SET(orelse, 0,
                          If(expression, suite_seq, suite_seq2,
-                            LINENO(CHILD(n, NCH(n) - 6)),
-                            CHILD(n, NCH(n) - 6)->n_col_offset,
+                            LINENO(CHILD(n, NCH(n) - 7)),
+                            CHILD(n, NCH(n) - 7)->n_col_offset,
                             end_lineno, end_col_offset, c->c_arena));
             /* the just-created orelse handled the last elif */
             n_elif--;