Bluetooth: A2DP: Check the pointer before using

Check the pointer `sep` before using it.

Fixes #74734.

(cherry picked from commit c9708ff)

Original-Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>
GitOrigin-RevId: c9708ff
Change-Id: Iccfdc1076259d05077c96921eaf590238392cc38
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5680119
Commit-Queue: 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>

subsys/bluetooth/host/classic/a2dp.c

@@ -195,11 +195,10 @@ static int a2dp_set_config_ind(struct bt_avdtp *session, struct bt_avdtp_sep *se
  int err;
 
  *errcode = 0;
+
+ __ASSERT(sep, "Invalid sep");
+
  ep = CONTAINER_OF(sep, struct bt_a2dp_ep, sep);
- if (ep == NULL) {
- *errcode = BT_AVDTP_BAD_ACP_SEID;
- return -1;
- }
 
  /* parse the configuration */
  codec_info_element_len = 4U;