Don't write to inferior_ptid in darwin-nat.c

Untested.

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

	* darwin-nat.c (darwin_nat_target::decode_message): Don't write to
	inferior_ptid.
	(darwin_nat_target::stop_inferior, darwin_nat_target::kill): Avoid
	inferior_ptid.
	(darwin_attach_pid): Use switch_to_no_thread instead of writing to
	inferior_ptid directly.
	(darwin_nat_target::init_thread_list): Switch to thread, instead
	of writing to inferior_ptid.
	(darwin_nat_target::attach): Don't write to inferior_ptid.
	(darwin_nat_target::get_ada_task_ptid): Avoid inferior_ptid.