[nrf fromtree] tests: bluetooth: tester: Fix NULL pointer dereference in error path

bt_conn_unref() requires valid conn pointer but could be called with
NULL in case valid connection was not found in disconnect_eatt_chans().

Fixes #39851

Signed-off-by: Szymon Janc <[email protected]>
(cherry picked from commit ef0e75134ef6463025256f0904c1957b72c10693)
Signed-off-by: Robert Lubos <[email protected]>

Author: sjanc

tests/bluetooth/tester/src/l2cap.c

@@ -344,22 +344,22 @@ void disconnect_eatt_chans(uint8_t *data, uint16_t len)
 	if (!conn) {
 		LOG_ERR("Unknown connection");
 		status = BTP_STATUS_FAILED;
-		goto rsp;
+		goto failed;
 	}
 
 	for (int i = 0; i < cmd->count; i++) {
 		err = bt_eatt_disconnect_one(conn);
 		if (err) {
 			status = BTP_STATUS_FAILED;
-			goto rsp;
+			goto unref;
 		}
 	}
 
 	status = BTP_STATUS_SUCCESS;
 
-rsp:
+unref:
 	bt_conn_unref(conn);
-
+failed:
 	tester_rsp(BTP_SERVICE_ID_L2CAP, L2CAP_DISCONNECT_EATT_CHANS,
 		   CONTROLLER_INDEX, status);
 }