Just return if nmemb is 0. Avoids a NULL dereference and is

consistent with the behavior of the other libc sort functions.
OK deraadt@

src/lib/libc/stdlib/merge.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: merge.c,v 1.9 2011/03/06 00:55:38 deraadt Exp $ */
+/*	$OpenBSD: merge.c,v 1.10 2015/06/21 03:20:56 millert Exp $ */
 /*-
  * Copyright (c) 1992, 1993
  *	The Regents of the University of California.  All rights reserved.
@@ -101,6 +101,9 @@ mergesort(void *base, size_t nmemb, size_t size,
 		return (-1);
 	}
 
+	if (nmemb == 0)
+		return (0);
+
 	/*
 	 * XXX
 	 * Stupid subtraction for the Cray.