Fix the posix_memalign() changes in r196861 to actually return a NULL pointer

as intended.

PR:		standards/138307

lib/libc/stdlib/malloc.c

@@ -5340,6 +5340,7 @@ posix_memalign(void **memptr, size_t alignment, size_t size)
 				size = 1;
 			else {
 				result = NULL;
+				*memptr = NULL;
 				ret = 0;
 				goto RETURN;
 			}