runtime: better panic for send to nil channel

*Much* better on NaCl, where memory faults are deadly.

R=r
CC=golang-dev
https://golang.org/cl/2249041

src/pkg/runtime/chan.c

@@ -403,6 +403,9 @@ void
 	int32 o;
 	byte *ae;
 
+	if(c == nil)
+		panicstring("send to nil channel");
+
 	o = rnd(sizeof(c), c->elemalign);
 	ae = (byte*)&c + o;
 	chansend(c, ae, nil);
@@ -416,6 +419,9 @@ void
 	int32 o;
 	byte *ae, *ap;
 
+	if(c == nil)
+		panicstring("send to nil channel");
+
 	o = rnd(sizeof(c), c->elemalign);
 	ae = (byte*)&c + o;
 	o = rnd(o+c->elemsize, Structrnd);