Fix #196 drop external file on playlist of empty project.

The single or first item should be opened in player automatically when
dropped with an empty project. Because it did not, the user could drag
directly from playlist to timeline, which would cause a crash.

src/docks/playlistdock.cpp

@@ -377,6 +377,8 @@ void PlaylistDock::onDropped(const QMimeData *data, int row)
             QString path = MAIN.removeFileScheme(url);
             Mlt::Producer p(MLT.profile(), path.toUtf8().constData());
             if (p.is_valid()) {
+                if (!MLT.producer())
+                    MAIN.open(path);
                 // Convert avformat to avformat-novalidate so that XML loads faster.
                 if (!qstrcmp(p.get("mlt_service"), "avformat")) {
                     p.set("mlt_service", "avformat-novalidate");