Use gdbpy_ref to simplify some logic

This uses the new gdbpy_ref template to simplify logic in various
parts of the Python layer; for example removing repeated error code or
removing gotos.

gdb/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* python/py-cmd.c (cmdpy_destroyer): Use gdbpy_ref.
	* python/py-breakpoint.c (gdbpy_breakpoint_deleted): Use
	gdbpy_ref.
	* python/py-type.c (field_new): Use gdbpy_ref.
	* python/py-symtab.c (symtab_and_line_to_sal_object): Use
	gdbpy_ref.
	* python/py-progspace.c (pspy_new): Use gdbpy_ref.
	(py_free_pspace): Likewise.
	(pspace_to_pspace_object): Likewise.
	* python/py-objfile.c (objfpy_new): Use gdbpy_ref.
	(py_free_objfile): Likewise.
	(objfile_to_objfile_object): Likewise.
	* python/py-inferior.c (delete_thread_object): Use
	gdbpy_ref.
	(infpy_read_memory): Likewise.
	(py_free_inferior): Likewise.
	* python/py-evtregistry.c (create_eventregistry_object): Use
	gdbpy_ref.
	* python/py-event.c (create_event_object): Use gdbpy_ref.