Make jump_to_playing_song not runnable if there is no playing song

NEWS

@@ -19,6 +19,7 @@ ncmpcpp-0.8 (????-??-??)
 * Lyrics and last_fm can now be startup screens and are lockable.
 * Action 'update_environment' now also synchronizes status with MPD.
 * Fixed an issue that could cause some MPD events to be missed.
+* ACtion 'jump_to_playing_song' is not runnable now if there is no playing song.
 
 ncmpcpp-0.7.7 (2016-10-31)
 * Fixed compilation on 32bit platforms.

src/actions.cpp

@@ -1204,27 +1204,26 @@ void UpdateDatabase::run()
 
 bool JumpToPlayingSong::canBeRun()
 {
-	return myScreen == myPlaylist
+	m_song = myPlaylist->nowPlayingSong();
+	return !m_song.empty()
+		&& (myScreen == myPlaylist
 		    || myScreen == myBrowser
-	    || myScreen == myLibrary;
+		    || myScreen == myLibrary);
 }
 
 void JumpToPlayingSong::run()
 {
-	auto s = myPlaylist->nowPlayingSong();
-	if (s.empty())
-		return;
 	if (myScreen == myPlaylist)
 	{
-		myPlaylist->locateSong(s);
+		myPlaylist->locateSong(m_song);
 	}
 	else if (myScreen == myBrowser)
 	{
-		myBrowser->locateSong(s);
+		myBrowser->locateSong(m_song);
 	}
 	else if (myScreen == myLibrary)
 	{
-		myLibrary->locateSong(s);
+		myLibrary->locateSong(m_song);
 	}
 }
 

src/actions.h

@@ -682,6 +682,8 @@ struct JumpToPlayingSong: BaseAction
 private:
 	virtual bool canBeRun() override;
 	virtual void run() override;
+
+	MPD::Song m_song;
 };
 
 struct ToggleRepeat: BaseAction