wifi: mac80211: check link exists before use

If the assoc link doesn't exist yet at this point, we
cannot use it yet. This isn't normally the case, but
e.g. in case of FT-DS (or just broken userspace) the
link might not be set up yet and will only be created
later in this function. Check that the link exists.

Fixes: 310c8387

 ("wifi: mac80211: clean up connection process")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/3151f5d0-c18f-413d-b34b-b94f095b947c@moroto.mountain
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240301193633.c886d300e90a.Ie1d5e23b2a033d934d343c37249f6f4dfddcc5fe@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>