wifi: iwlwifi: mld: add NULL check for channel in DW end handler

wifi: iwlwifi: mld: add NULL check for channel in DW end handler

ieee80211_get_channel() can return NULL if the frequency is not
registered in the wiphy (e.g. due to regulatory domain restrictions).
The returned channel pointer is passed directly to
cfg80211_next_nan_dw_notif() which dereferences it unconditionally
in both the tracepoint and the netlink message, causing a NULL
pointer dereference.

Add a NULL check before using the channel pointer.

Signed-off-by: Daniel Gabay <daniel.gabay@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Link: https://patch.msgid.link/20260512222731.a250203cd1c6.I1d807aab415da30a55dd89a974c3226adc547ebb@changeid
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>