Use Dv for NULL.

lib/libc/stdlib/posix_memalign.3

@@ -1,4 +1,4 @@
-.\"	$NetBSD: posix_memalign.3,v 1.2 2015/11/07 16:21:42 nros Exp $
+.\"	$NetBSD: posix_memalign.3,v 1.3 2015/11/07 18:47:26 wiz Exp $
 .\"
 .\" Copyright (C) 2006 Jason Evans <jasone@FreeBSD.org>.
 .\" All rights reserved.
@@ -49,7 +49,7 @@ The
 function allocates
 .Fa size
 bytes of memory such that the allocation's base address is an even multiple of
-.Fa alignment , 
+.Fa alignment ,
 and returns the allocation in the value pointed to by
 .Fa ptr .
 The requested
@@ -86,7 +86,9 @@ function returns the value 0 if successful; otherwise it returns an error value.
 The
 .Fn aligned_alloc
 function returns a pointer to the allocated memory if successful; on failure it
-returns NULL and sets
+returns
+.Dv NULL
+and sets
 .Fa errno
 to indicate the error.
 .Sh ERRORS