wifi: mac80211: Don't finalize CSA in IBSS mode if state is disconnected

In the Linux kernel, the following vulnerability has been resolved:

wifi: mac80211: Don't finalize CSA in IBSS mode if state is disconnected

When we are not connected to a channel, sending channel "switch"

announcement doesn't make any sense.

The BSS list is empty in that case. This causes the for loop in

cfg80211_get_bss() to be bypassed, so the function returns NULL

(check line 1424 of net/wireless/scan.c), causing the WARN_ON()

in ieee80211_ibss_csa_beacon() to get triggered (check line 500

of net/mac80211/ibss.c), which was consequently reported on the

syzkaller dashboard.

Thus, check if we have an existing connection before generating

the CSA beacon in ieee80211_ibss_finish_csa().