libc: Fix null pointer arithmetic warning in mergesort

This file has other questionable code and "optimizations" (such as copying
one int at a time) that are probably no longer useful, so it might make
sense to replace it with a different implementation at some point.

Reviewed By:	jhb
Differential Revision: https://reviews.freebsd.org/D28134

lib/libc/stdlib/merge.c

@@ -52,7 +52,7 @@ __FBSDID("$FreeBSD$");
  * (The default is pairwise merging.)
  */
 
-#include <sys/types.h>
+#include <sys/param.h>
 
 #include <errno.h>
 #include <stdlib.h>
@@ -97,9 +97,7 @@ static void insertionsort(u_char *, size_t, size_t, cmp_t);
  * boundaries.
  */
 /* Assumption: PSIZE is a power of 2. */
-#define EVAL(p) (u_char **)						\
-	((u_char *)0 +							\
-	    (((u_char *)p + PSIZE - 1 - (u_char *) 0) & ~(PSIZE - 1)))
+#define EVAL(p) (u_char **)roundup2((uintptr_t)p, PSIZE)
 
 #ifdef I_AM_MERGESORT_B
 int mergesort_b(void *, size_t, size_t, cmp_t);