EDMG: Validate pri channel lookup result before using it

At least in theory, hw_get_channel_freq() could return NULL, so add
error handling for that.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

src/ap/hw_features.c

@@ -838,6 +838,8 @@ static int hostapd_is_usable_edmg(struct hostapd_iface *iface)
 				       iface->freq, NULL,
 				       iface->hw_features,
 				       iface->num_hw_features);
+	if (!pri_chan)
+		return 0;
 	hostapd_encode_edmg_chan(iface->conf->enable_edmg,
 				 iface->conf->edmg_channel,
 				 pri_chan->chan,