Use gdbpy_ref in archpy_disassemble

This changes archpy_disassemble to use gdbpy_ref.  It also fixes a
latent bug where archpy_disassemble was decref'ing the results of a
all to PyArg_ParseTupleAndKeywords.  This is incorrect because
PyArg_ParseTupleAndKeywords returns borrowed references.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-arch.c (archpy_disassemble): Use gdbpy_ref.  Don't
	decref results of PyArg_ParseTupleAndKeywords.