Unlock the playlist while waiting for input to finish so that input can freely...

Unlock the playlist while waiting for input to finish so that input can freely use signal on the playlist whatever the context is (Closes:#937)

src/playlist/engine.c

@@ -291,7 +291,9 @@ check_input:
         else if( p_playlist->p_input->b_die )
         {
             PL_DEBUG( "dying input" );
+            PL_UNLOCK;
             msleep( 25000 ); // 25 ms
+            PL_LOCK;
             goto check_input;
         }
         /* This input has finished, ask it to die ! */