runtime: check for nil value pointer in select syncsend case

Fixes #2309.

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5128053

src/pkg/runtime/chan.c

@@ -1024,6 +1024,7 @@ syncsend:
 	selunlock(sel);
 	if(debug)
 		runtime·printf("syncsend: sel=%p c=%p o=%d\n", sel, c, o);
+	if(sg->elem != nil)
 		c->elemalg->copy(c->elemsize, sg->elem, cas->sg.elem);
 	gp = sg->g;
 	gp->param = sg;

test/chan/select7.go

@@ -0,0 +1,68 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test select when discarding a value.
+
+package main
+
+import "runtime"
+
+func recv1(c <-chan int) {
+	<-c
+}
+
+func recv2(c <-chan int) {
+	select {
+	case <-c:
+	}
+}
+
+func recv3(c <-chan int) {
+	c2 := make(chan int)
+	select {
+	case <-c:
+	case <-c2:
+	}
+}
+
+func send1(recv func(<-chan int)) {
+	c := make(chan int)
+	go recv(c)
+	runtime.Gosched()
+	c <- 1
+}
+
+func send2(recv func(<-chan int)) {
+	c := make(chan int)
+	go recv(c)
+	runtime.Gosched()
+	select {
+	case c <- 1:
+	}
+}
+
+func send3(recv func(<-chan int)) {
+	c := make(chan int)
+	go recv(c)
+	runtime.Gosched()
+	c2 := make(chan int)
+	select {
+	case c <- 1:
+	case c2 <- 1:
+	}
+}
+
+func main() {
+	send1(recv1)
+	send2(recv1)
+	send3(recv1)
+	send1(recv2)
+	send2(recv2)
+	send3(recv2)
+	send1(recv3)
+	send2(recv3)
+	send3(recv3)
+}