Don't write to inferior_ptid in corelow.c

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

	* corelow.c (core_target::close): Use switch_to_no_thread instead
	of writing to inferior_ptid directly.
	(add_to_thread_list, core_target_open): Use switch_to_thread
	instead of writing to inferior_ptid directly.