gst/gstbuffer.c (gst_buffer_get_caps): Like all our _get accessors returning refcounted objects, return a ref.

Original commit message from CVS:
2005-09-22  Andy Wingo  <wingo@pobox.com>

* gst/gstbuffer.c (gst_buffer_get_caps): Like all our _get
accessors returning refcounted objects, return a ref.

* check/gst/gstbuffer.c (GST_START_TEST): Use refcount-idempotent
accessor for caps. IDEMPOTENCE. Oh yes.