wifi: mac80211: mlme: fix null-ptr deref on failed assoc

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

wifi: mac80211: mlme: fix null-ptr deref on failed assoc

If association to an AP without a link 0 fails, then we crash in
tracing because it assumes that either ap_mld_addr or link 0 BSS
is valid, since we clear sdata->vif.valid_links and then don't
add the ap_mld_addr to the struct.

Since we clear also sdata->vif.cfg.ap_addr, keep a local copy of
it and assign it earlier, before clearing valid_links, to fix
this.