Don't write to inferior_ptid in tracefile-tfile.c

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* tracefile-tfile.c (tfile_target_open): Don't write to
	inferior_ptid directly, instead switch to added thread.
	(tfile_target::close): Use switch_to_no_thread instead of writing
	to inferior_ptid directly.