modem: cmux: Prevent NULL deref in dlci_open_handler

Prevent unlikely item == NULL scenario (CID 323102) and remove
superflous cmux == NULL check (CID 334903)

(cherry picked from commit cc3076b)

Original-Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>
GitOrigin-RevId: cc3076b
Change-Id: I910d3e00bac478894de13a48380d0f3ffb50dfbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5299387
Tested-by: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>

subsys/modem/modem_cmux.c

@@ -1015,13 +1015,16 @@ struct modem_pipe_api modem_cmux_dlci_pipe_api = {
 
 static void modem_cmux_dlci_open_handler(struct k_work *item)
 {
-	struct k_work_delayable *dwork = k_work_delayable_from_work(item);
-	struct modem_cmux_dlci *dlci = CONTAINER_OF(dwork, struct modem_cmux_dlci, open_work);
+	struct k_work_delayable *dwork;
+	struct modem_cmux_dlci *dlci;
 
-	if (dlci == NULL) {
+	if (item == NULL) {
 		return;
 	}
 
+	dwork = k_work_delayable_from_work(item);
+	dlci = CONTAINER_OF(dwork, struct modem_cmux_dlci, open_work);
+
 	dlci->state = MODEM_CMUX_DLCI_STATE_OPENING;
 
 	struct modem_cmux_frame frame = {