carl9170: tx: fix an incorrect use of list iterator

If the previous list_for_each_entry_continue_rcu() don't exit early
(no goto hit inside the loop), the iterator 'cvif' after the loop
will be a bogus pointer to an invalid structure object containing
the HEAD (&ar->vif_list). As a result, the use of 'cvif' after that
will lead to a invalid memory access (i.e., 'cvif->id': the invalid
pointer dereference when return back to/after the callsite in the
carl9170_update_beacon()).

The original intention should have been to return the valid 'cvif'
when found in list, NULL otherwise. So just return NULL when no
entry found, to fix this bug.

Cc: stable@vger.kernel.org
Fixes: 1f1d9654

 ("carl9170: refactor carl9170_update_beacon")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220328122820.1004-1-xiam0nd.tong@gmail.com