adaptivedemux: Actually wait for playlist update when requested

adaptivedemux: Actually wait for playlist update when requested

There are several cases where a HLS server could temporarily have wrong
fragments, or reconfigure the playlist. In those cases, when we get
fragment download failures, we *really* want to wait a bit (for the next
playlist update) before retrying to get fragments.

Previously this method was first checking to see if there was next fragments
(according to the previous manifest update) before waiting for the next update.
The problem was that if that if there is a temporary failure on the server,
that's uncorrelated to whether the manifest contains next fragments or not.