cmd/gc: replace NULL by nil

In CL 4050, NULL was used instead of nil.
However, Plan 9 doesn't declare NULL.

Change-Id: I8295a3102509a1ce417278f23a37cbf65938cce1
Reviewed-on: https://go-review.googlesource.com/4814
Reviewed-by: Minux Ma <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

src/cmd/gc/walk.c

@@ -634,7 +634,7 @@ walkexpr(Node **np, NodeList **init)
 
 			// Append captured variables to argument list.
 			n->list = concat(n->list, n->left->enter);
-			n->left->enter = NULL;
+			n->left->enter = nil;
 			// Replace OCLOSURE with ONAME/PFUNC.
 			n->left = n->left->closure->nname;
 			// Update type of OCALLFUNC node.