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>