httplive: Adjust conditions for playlist reloading

Commit 7655d6c889d2425c1fd4615f7e2692df1a47470c changes HLS http
playlist reloading policy. But leaving only one fragment in buffer is
not enough. This patch ensures that less than three fragments in buffer
leads to playlist reloading. The overal result is more reliable http
streams.

Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>