cmd/gc: replace NULL by nil

In CL 3964, NULL was used instead of nil.
However, Plan 9 doesn't declare NULL.

Change-Id: Ied3850aca5c8bca5974105129a37d575df33f6ec
Reviewed-on: https://go-review.googlesource.com/5150Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/gc/closure.c

@@ -140,8 +140,8 @@ closurename(Node *n)
 	if(n->sym != S)
 		return n->sym;
 	gen = 0;
-	outer = NULL;
-	prefix = NULL;
+	outer = nil;
+	prefix = nil;
 	if(n->outerfunc == N) {
 		// Global closure.
 		outer = "glob";