runtime: always set *received in chanrecv.

Also fix comment.

The only caller of chanrecv initializes the value to false, so
this patch makes no difference at present.  But it seems like
the right thing to do.

R=rsc
CC=golang-dev
https://golang.org/cl/4312053

src/pkg/runtime/chan.c

@@ -366,6 +366,8 @@ asynch:
 		if(selected != nil) {
 			runtime·unlock(c);
 			*selected = false;
+			if(received != nil)
+				*received = false;
 			return;
 		}
 		sg = allocsg(c);
@@ -521,7 +523,7 @@ runtime·selectnbrecv(byte *v, Hchan *c, bool selected)
 // compiler implements
 //
 //	select {
-//	case v = <-c:
+//	case v, ok = <-c:
 //		... foo
 //	default:
 //		... bar
@@ -529,7 +531,7 @@ runtime·selectnbrecv(byte *v, Hchan *c, bool selected)
 //
 // as
 //
-//	if c != nil && selectnbrecv(&v, c) {
+//	if c != nil && selectnbrecv2(&v, &ok, c) {
 //		... foo
 //	} else {
 //		... bar