gst/asfdemux/gstasfdemux.*: Don't crash in the seek event handling code when playtime...

gst/asfdemux/gstasfdemux.*: Don't crash in the seek event handling code when playtime is 0, as may be the case with l...

Original commit message from CVS:
* gst/asfdemux/gstasfdemux.c: (gst_asf_demux_handle_seek_event),
(gst_asf_demux_process_data), (gst_asf_demux_process_file),
(gst_asf_demux_handle_src_query), (gst_asf_demux_change_state):
* gst/asfdemux/gstasfdemux.h:
Don't crash in the seek event handling code when playtime is 0,
as may be the case with live streams (#386218). Implement SEEKING
query so applications can query seekability without second-guessing
based on whether we have a duration or not.