Fix several misuse of gst_buffer_merge (it doesn't take ownership of any buffer), should fix some leaks. I hope I did...

Original commit message from CVS:
2004-03-06  Christophe Fergeau  <teuf@gnome.org>

* ext/faac/gstfaac.c: (gst_faac_chain):
* ext/flac/gstflactag.c: (gst_flac_tag_chain):
* ext/libpng/gstpngenc.c: (user_write_data):
* ext/mikmod/gstmikmod.c: (gst_mikmod_loop):
* gst/ac3parse/gstac3parse.c: (gst_ac3parse_chain):
* gst/mpeg2sub/gstmpeg2subt.c: (gst_mpeg2subt_chain_subtitle):
* gst/mpegstream/gstrfc2250enc.c: (gst_rfc2250_enc_add_slice):
Fix several misuse of gst_buffer_merge (it doesn't take ownership
of any buffer), should fix some leaks. I hope I didn't unref buffers
that shouldn't be...