Bluetooth: A2DP: Fix NULL pointer references issue

The opposite logic is used to determine if a
pointer is valid.

Correct the judgment logic.

Fixes #74728.

(cherry picked from commit 8276d4f)

Original-Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>
GitOrigin-RevId: 8276d4f
Change-Id: I1181bb67de74a3b7003b4f1ad3b894365faa6801
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5680118
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>

subsys/bluetooth/host/classic/a2dp.c

@@ -725,7 +725,7 @@ int bt_a2dp_stream_establish(struct bt_a2dp_stream *stream)
 
  a2dp = stream->a2dp;
  a2dp->open_param.req.func = bt_a2dp_open_cb;
- a2dp->open_param.acp_stream_ep_id = stream->remote_ep == NULL ?
+ a2dp->open_param.acp_stream_ep_id = stream->remote_ep != NULL ?
  stream->remote_ep->sep.sep_info.id : stream->remote_ep_id;
  a2dp->open_param.sep = a2dp->set_config_param.sep;
  return bt_avdtp_open(&a2dp->session, &a2dp->open_param);