gst/asfdemux/gstasfdemux.c: Align by packetsize, and assert that we a packet available before playing. The first make...

Original commit message from CVS:
* gst/asfdemux/gstasfdemux.c: (gst_asf_demux_loop):
Align by packetsize, and assert that we a packet available before
playing. The first makes webstreams work (they often include
trailing padding data in a packet), the second allows pausing a
ASF stream in totem without getting demux errors afterwards.