Safeguard against a null pipeline in GstEngine::Play.

src/engines/enginebase.cpp

@@ -78,3 +78,9 @@ void Engine::Base::EmitAboutToEnd() {
 int Engine::Base::AddBackgroundStream(const QUrl& url) {
   return -1;
 }
+
+bool Engine::Base::Play(const QUrl& u, TrackChangeType c) {
+  if (!Load(u, c))
+    return false;
+  return Play();
+}

src/engines/enginebase.h

@@ -61,7 +61,7 @@ class Base : public QObject, boost::noncopyable {
 
   // Helpers
   virtual bool Load(const QUrl &url, TrackChangeType change);
-  bool Play(const QUrl &u, TrackChangeType c) { return Load(u, c) && Play(); }
+  bool Play(const QUrl &u, TrackChangeType c);
   void SetVolume( uint value );
 
 

src/engines/gstengine.cpp

@@ -496,6 +496,9 @@ void GstEngine::StartFadeout() {
 bool GstEngine::Play( uint offset ) {
   EnsureInitialised();
 
+  if (!current_pipeline_)
+    return false;
+
   QFuture<GstStateChangeReturn> future = current_pipeline_->SetState(GST_STATE_PLAYING);
   BoundFutureWatcher<GstStateChangeReturn, uint>* watcher =
       new BoundFutureWatcher<GstStateChangeReturn, uint>(offset, this);