WNM: Check wpa_s->current_bss more consistently

The scan result comparison routine would not make much sense without
current BSS level known, so return from the function without going
through the iteration that could have dereferenced the pointer if
wpa_s->current_bss == NULL.

CRs-Fixed: 684325
Change-Id: Ifb12fb53f5ca193e0d1c23c3115e3507108cbf00
Git-commit: 67adcd266c4ef3f9356190ef2c0727303f9532cb
Git-repo : git://w1.fi/srv/git/hostap.git
Signed-off-by: Jouni Malinen <j@w1.fi>