httplive: playlist reload not frequent enough.

Looks like after commit 7655d6c8 I have bad HLS HTTP streams viewing experience. Looks like player skips big fragments of data. Looks like the goal of this commit is reduce playlist reloading. It reloads playlist only if there are only one fragment in buffer. For me practice shows that this can lead to loss of one fragment next to those in buffer. The simple patch increases "critical" amount data in buffer and can make viewing of HTTP stream more reliable. But i have little experience in HLS HTTP streams. So I'd like to have more professional solution.