gc: const nil bug

Fixes #1073.

R=ken2
CC=golang-dev
https://golang.org/cl/2169043

src/cmd/gc/walk.c

@@ -261,6 +261,10 @@ walkdef(Node *n)
 			yyerror("const initializer must be constant");
 			goto ret;
 		}
+		if(isconst(e, CTNIL)) {
+			yyerror("const initializer cannot be nil");
+			goto ret;
+		}
 		t = n->type;
 		if(t != T) {
 			convlit(&e, t);

test/const1.go

@@ -77,3 +77,5 @@ func main() {
 	f(String)	// ERROR "convert|wrong type|cannot|incompatible"
 	f(Bool)	// ERROR "convert|wrong type|cannot|incompatible"
 }
+
+const ptr = nil  // ERROR "const.*nil"