gst/: Post an error message when we get an EOS event and were not able to find out the type of stream.

Original commit message from CVS:
* gst/apetag/gsttagdemux.c: (gst_tag_demux_init),
(gst_tag_demux_sink_event):
* gst/id3demux/gstid3demux.c: (gst_id3demux_init),
(gst_id3demux_sink_event):
Post an error message when we get an EOS event and were not
able to find out the type of stream.
* tests/check/elements/id3v2mux.c: (fill_mp3_buffer), (got_buffer),
(test_taglib_id3mux_with_tags):
Decrease num-buffers to 16 per iteration again, otherwise the
many memcpy()s and reallocations in the test will hammer slow
CPUs completely and make the test timeout.