rtw88: fix null vif pointer when hw_scan fails

Add this check to avoid crash by dereferencing a null pointer. When hwscan
fails due to no memory or dma failure, the scan flag in ieee80211_local is
cleared. So mac80211 determine that it's not hw_scan then calls
sw_scan_complete() with null vif, which is also freed during the fail.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220520081523.45987-1-pkshih@realtek.com