runtime: better error for send/recv on nil channel

R=ken2
CC=golang-dev
https://golang.org/cl/1835041

src/pkg/runtime/chan.c

@@ -182,6 +182,9 @@ chansend(Hchan *c, byte *ep, bool *pres)
 	SudoG *sg;
 	G* gp;
 
+	if(c == nil)
+		panicstring("send to nil channel");
+
 	if(gcwaiting)
 		gosched();
 
@@ -286,6 +289,9 @@ chanrecv(Hchan* c, byte *ep, bool* pres)
 	SudoG *sg;
 	G *gp;
 
+	if(c == nil)
+		panicstring("receive from nil channel");
+
 	if(gcwaiting)
 		gosched();