avoid indirection through nil pointer by not using p->wired->machno.

Reference: /n/atom/patch/applied/9wiredmachp
Date: Mon Jun 23 18:39:44 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/port/proc.c	Mon Jun 23 18:35:16 2014
+++ /sys/src/9/port/proc.c	Mon Jun 23 18:35:16 2014
@@ -404,7 +404,7 @@
 		return;
 	}
 
-	if(up != p && (p->wired == nil || p->wired->machno == m->machno))
+	if(up != p && (p->wired == nil || p->wired == MACHP(m->machno)))
 		m->readied = p;	/* group scheduling */
 
 	updatecpu(p);
@@ -490,7 +490,7 @@
 
 	/* cooperative scheduling until the clock ticks */
 	if((p=m->readied) && p->mach==0 && p->state==Ready
-	&& (p->wired == nil || p->wired->machno == m->machno)
+	&& (p->wired == nil || p->wired == MACHP(m->machno))
 	&& runq[Nrq-1].head == nil && runq[Nrq-2].head == nil){
 		skipscheds++;
 		rq = &runq[p->priority];