make_cleanup_restore_current_thread: Look up thread earlier

make_cleanup_restore_current_thread: Look up thread earlier

The unconditional is_stopped call already asserts that the thread exists.

gdb/ChangeLog:
2017-05-04  Pedro Alves  <palves@redhat.com>

	* thread.c (make_cleanup_restore_current_thread): Move
	find_thread_ptid call before the is_stopped call.  Assert that the
	thread is found.  Replace is_stopped call by checking the thread's
	state directly.  Remove unnecessary NULL-thread check.