net: ieee802154: Fix possible null pointer de-reference

ieee802154_scan() checks if ctx->scan_ctx (scan) is NULL what implies
that this can be true, but de-reference this variable before this
check what may cause a problem.

Fixes zephyrproject-rtos#23299 [3]

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

subsys/net/l2/ieee802154/ieee802154_mgmt.c

@@ -97,6 +97,10 @@ static int ieee802154_scan(u32_t mgmt_request, struct net_if *iface,
 		return -EALREADY;
 	}
 
+	if (scan == NULL) {
+		return -EINVAL;
+	}
+
 	if (mgmt_request == NET_REQUEST_IEEE802154_ACTIVE_SCAN) {
 		struct ieee802154_frame_params params;