replay: move onto NULL check before first use

replay: move onto NULL check before first use

cmd_replay() aborts if the pointer "onto" is NULL after argument
parsing, e.g. when specifying a non-existing commit with --onto.
15cd4ef1f4 (replay: make atomic ref updates the default behavior,
2025-11-06) added code that dereferences this pointer before the check.
Switch their places to avoid a segmentation fault.

Reported-by: Kristoffer Haugsbakk <kristofferhaugsbakk@fastmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>