avoid left shift overflow in reallocarray.

Some 64-bit platforms (e.g. Windows 64) have a 32-bit long. So, shifting
1UL 32-bits to the left causes an overflow. This replaces the constant 1UL with
(size_t)1 so that we get the correct constant size for the platform.
discussed with tedu@ & deraadt@

src/lib/libc/stdlib/reallocarray.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: reallocarray.c,v 1.1 2014/05/08 21:43:49 deraadt Exp $	*/
+/*	$OpenBSD: reallocarray.c,v 1.2 2014/12/08 03:45:00 bcook Exp $	*/
 /*
  * Copyright (c) 2008 Otto Moerbeek <otto@drijf.net>
  *
@@ -24,7 +24,7 @@
  * This is sqrt(SIZE_MAX+1), as s1*s2 <= SIZE_MAX
  * if both s1 < MUL_NO_OVERFLOW and s2 < MUL_NO_OVERFLOW
  */
-#define MUL_NO_OVERFLOW	(1UL << (sizeof(size_t) * 4))
+#define MUL_NO_OVERFLOW	((size_t)1 << (sizeof(size_t) * 4))
 
 void *
 reallocarray(void *optr, size_t nmemb, size_t size)