Port xsparse.c to AIX

* scripts/xsparse.c (emalloc): Do not report failure when malloc
(0) returns NULL, as it does on AIX.  Simply return a null
pointer; that’s good enough for xsparse.c.

scripts/xsparse.c

@@ -65,7 +65,7 @@ static void *
 emalloc (size_t size)
 {
   char *p = malloc (size);
-  if (!p)
+  if (!p && size)
     die (1, "not enough memory");
   return p;
 }