This changes more places in python.c to use gdbpy_ref.
Additionally, previously gdbpy_apply_type_printers would return
EXT_LANG_RC_ERROR if a type printer returned None. However, that
doesn't seem correct to me; this patch changes it to return
EXT_LANG_RC_NOP in this case.
2017-01-10 Tom Tromey <tom@tromey.com>
* python/python.c (eval_python_command, gdbpy_decode_line)
(gdbpy_run_events, gdbpy_start_type_printers)
(gdbpy_apply_type_printers): Use gdbpy_ref.