Change python_run_simple_file to use gdbpy_ref

Change python_run_simple_file to use gdbpy_ref

This changes python_run_simple_file to use gdbpy_ref and
unique_xmalloc_ptr.  Thi fixes a latent bug in this function, where
the error path previously ran the cleanups and then referred to one of
the objects just freed.

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

	* python/python.c (python_run_simple_file): Use
	unique_xmalloc_ptr, gdbpy_ref.