This changes objfile_to_objfile_object to return a new references and
fixes up all the uses.

gdb/ChangeLog
2018-09-16  Tom Tromey  <tom@tromey.com>

	* python/py-progspace.c (pspy_get_objfiles): Update.
	* python/python-internal.h (objfile_to_objfile_object): Change
	return type.
	* python/py-newobjfileevent.c (create_new_objfile_event_object):
	Update.
	* python/py-xmethods.c (gdbpy_get_matching_xmethod_workers):
	Update.
	* python/python.c (gdbpy_get_current_objfile): Update.
	(gdbpy_objfiles): Update.
	* python/py-objfile.c (objfpy_get_owner, gdbpy_lookup_objfile):
	Update.
	(objfile_to_objfile_object): Return a new reference.
	* python/py-symtab.c (stpy_get_objfile): Update.
	* python/py-prettyprint.c (find_pretty_printer_from_objfiles):
	Update.