Merge pull request #2171 from NaterGator/master

Fix potential null pointer deref in Mavlink dtor if task_main returns error

src/modules/mavlink/mavlink_main.cpp

@@ -249,7 +249,9 @@ Mavlink::~Mavlink()
 		} while (_task_running);
 	}
 
-	LL_DELETE(_mavlink_instances, this);
+	if (_mavlink_instances) {
+		LL_DELETE(_mavlink_instances, this);
+	}
 }
 
 void