avfilter/vf_fps: make sure the fifo is not empty before using it

Fixes Ticket2905
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_fps.c

@@ -205,7 +205,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
     }
 
     /* now wait for the next timestamp */
-    if (buf->pts == AV_NOPTS_VALUE) {
+    if (buf->pts == AV_NOPTS_VALUE || av_fifo_size(s->fifo) <= 0) {
         return write_to_fifo(s->fifo, buf);
     }