ffmpeg: Free last_frame instead of just unref

Fixes Ticket4611
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1178,6 +1178,8 @@ static void do_video_out(AVFormatContext *s,
     av_frame_unref(ost->last_frame);
     if (next_picture)
         av_frame_ref(ost->last_frame, next_picture);
+    else
+        av_frame_free(&ost->last_frame);
 }
 
 static double psnr(double d)