Return gdbpy_ref from gdbpy_get_varobj_pretty_printer

Return gdbpy_ref from gdbpy_get_varobj_pretty_printer

This changes gdbpy_get_varobj_pretty_printer to return a gdbpy_ref.

gdb/ChangeLog
2018-11-04  Tom Tromey  <tom@tromey.com>

	* varobj.c (install_default_visualizer): Update.
	* python/python-internal.h (gdbpy_get_varobj_pretty_printer):
	Return gdbpy_ref.
	* python/py-prettyprint.c (search_pp_list): Return gdbpy_ref.
	(find_pretty_printer_from_progspace)
	(find_pretty_printer_from_gdb, find_pretty_printer)
	(gdbpy_get_varobj_pretty_printer): Return gdbpy_ref.
	(gdbpy_get_varobj_pretty_printer, gdbpy_default_visualizer):
	Update.