demux: adaptive: don't seek disabled streams

As it was always seeking disabled streams,
init chunk was read on seek, queuing possible es
changes which are uncommited due to absence of sample data.
following dac320e
the fakeesout was always in restarting state then,
causing an always failing setposition which now checks
the restarting state.

modules/demux/adaptive/PlaylistManager.cpp

@@ -247,7 +247,9 @@ bool PlaylistManager::setPosition(mtime_t time)
         std::vector<AbstractStream *>::iterator it;
         for(it=streams.begin(); it!=streams.end(); ++it)
         {
-            ret &= (*it)->setPosition(time, !real);
+            AbstractStream *st = *it;
+            if(!st->isDisabled())
+                ret &= st->setPosition(time, !real);
         }
         if(!ret)
             break;