runtime: zero dangling pointer

I don't see how it can lead to bad things today.
But it's better to kill it before it does.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://golang.org/cl/111130045

src/pkg/runtime/chan.goc

@@ -1105,6 +1105,8 @@ loop:
 	if(sgp == nil)
 		return nil;
 	q->first = sgp->link;
+	if(q->last == sgp)
+		q->last = nil;
 
 	// if sgp participates in a select and is already signaled, ignore it
 	if(sgp->selectdone != nil) {