Use gdbpy_ref in python.c

Use gdbpy_ref in python.c

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.