Change inferior_to_inferior_object to return a gdbpy_ref

Change inferior_to_inferior_object to return a gdbpy_ref

Most callers of inferior_to_inferior_object already use a gdbpy_ref,
so this changes inferior_to_inferior_object to return one.  Doing this
revealed that create_thread_object was not correctly handling the case
where inferior_to_inferior_object failed, so this patch fixes this as
well.

gdb/ChangeLog
2019-01-02  Tom Tromey  <tom@tromey.com>

	* python/python-internal.h (inferior_to_inferior_object): Change
	return type.
	* python/py-exitedevent.c (create_exited_event_object): Update.
	* python/py-inferior.c (inferior_to_inferior_object): Return
	gdbpy_ref.
	(python_new_inferior, python_inferior_deleted)
	(thread_to_thread_object, delete_thread_object)
	(build_inferior_list, gdbpy_selected_inferior): Update.
	* python/py-infthread.c (create_thread_object): Update.  Also fail
	if inferior_to_inferior_object fails.