In Load_ChainContextSubst2, handle the case where an empty class

In Load_ChainContextSubst2, handle the case where an empty class

Fri Mar 15 01:35:56 2002  Owen Taylor  <otaylor@redhat.com>

	* pango/opentype/ftxgsub.c pango/opentype/ftxopen.c
	pango/opentype/ftxopenf.h: In Load_ChainContextSubst2,
	handle the case where an empty class definition
	is represented by an offset of 0.

	* pango/opentype/ftxgpos.c: Same for Load_ChainContextPos2.

	* pango/opentype/{ftxopen.c,ftxgpos.c,ftxgsub.c,ftgdef.c}:
	Fix pervasive bug where on cleanups on failure of loading
	an array element, all array elements were freed, not
	just the ones that had been succesfully loaded.