Initialize td_sel in the thread_init(). Struct thread is not zeroed

on the initial allocation, but seltdinit() assumes that td_sel is NULL
or a valid pointer.  Note that thread_fini()/seltdfini() also relies
on this, but correctly resets td_sel to NULL.

Submitted by:	luke.tw@gmail.com
PR:	199518
MFC after:	1 week

sys/kern/kern_thread.c

@@ -209,6 +209,7 @@ thread_init(void *mem, int size, int flags)
 	td->td_sched = (struct td_sched *)&td[1];
 	umtx_thread_init(td);
 	td->td_kstack = 0;
+	td->td_sel = NULL;
 	return (0);
 }