hlsdemux: Do not clear/advance fragment by finished header downloading

Header data must be forwarded to downstream, but if demux does not finish
to finding type (e.g., ts, mp4 and etc), this header data can be cleared
by _stream_clear_pending_data(). Moreover, although demux finish downloading
header data, still it has fragment date to be downloaded, fragment sequence
shouldn't be advanced yet at that moment.

https://bugzilla.gnome.org/show_bug.cgi?id=776928

ext/hls/gsthlsdemux.c

@@ -947,6 +947,9 @@ gst_hls_demux_finish_fragment (GstAdaptiveDemux * demux,
     }
   }
 
+  if (G_UNLIKELY (stream->downloading_header || stream->downloading_index))
+    return GST_FLOW_OK;
+
   gst_hls_demux_stream_clear_pending_data (hls_stream);
 
   if (ret == GST_FLOW_OK || ret == GST_FLOW_NOT_LINKED)