qtnfmac: fix NULL pointer dereference

In case *vif* is NULL at 655: if (!vif), the execution path jumps to
label out, where *vif* is dereferenced at 679:

if (vif->sta_state == QTNF_STA_CONNECTING)

Fix this by immediately returning when *vif* is NULL instead of
jumping to label out.

Addresses-Coverity-ID: 1469567 ("Dereference after null check")
Fixes: 480daa9c ("qtnfmac: fix invalid STA state on EAPOL failure")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Sergey Matyukevich <sergey.matyukevich.os@quanenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>