FFplay was using a 5 frame packet buffer, this is not much (e.g. 200 ms for
25fps video), when HLS is requesting a new segment via HTTP, it may take longer
for the request to complete.

Should fix ticket #4720.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Marton Balint <cus@passwd.hu>