mavlink_receiver: check for allocation failure

Also remove the start() declration (there's no definition of that)

src/modules/mavlink/mavlink_receiver.cpp

@@ -2493,6 +2493,11 @@ void *MavlinkReceiver::start_helper(void *context)
 
 	MavlinkReceiver *rcv = new MavlinkReceiver((Mavlink *)context);
 
+	if (!rcv) {
+		PX4_ERR("alloc failed");
+		return nullptr;
+	}
+
 	void *ret = rcv->receive_thread(nullptr);
 
 	delete rcv;

src/modules/mavlink/mavlink_receiver.h

@@ -99,13 +99,6 @@ public:
 	 */
 	~MavlinkReceiver();
 
-	/**
-	* Start the mavlink task.
-	 *
-	 * @return		OK on success.
-	 */
-	int		start();
-
 	/**
 	 * Display the mavlink status.
 	 */