Bluetooth: A2DP: Fix NULL pointer references issue

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

Correct the judgment logic.

Fixes #74732.

(cherry picked from commit 6fad658)

Original-Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>
GitOrigin-RevId: 6fad658
Change-Id: I5395a10a449c6ff73dbd5d1b6406adaa0522f8ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5680117
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

@@ -747,7 +747,7 @@ int bt_a2dp_stream_start(struct bt_a2dp_stream *stream)
 
 	a2dp = stream->a2dp;
 	a2dp->start_param.req.func = bt_a2dp_start_cb;
-	a2dp->start_param.acp_stream_ep_id = stream->remote_ep == NULL ?
+	a2dp->start_param.acp_stream_ep_id = stream->remote_ep != NULL ?
 		stream->remote_ep->sep.sep_info.id : stream->remote_ep_id;
 	a2dp->start_param.sep = &stream->local_ep->sep;
 	return bt_avdtp_start(&a2dp->session, &a2dp->start_param);