janitor: use NULL and not 0 for pointers.

Brought to you thanks to coccinelle:

---8<----
@@
expression *E;
@@
(
E ==
- 0
+ NULL
|
E !=
- 0
+ NULL
|
E =
- 0
+ NULL
)

@@
identifier f;
type T;
@@
T *f(...) {
<...
- return 0;
+ return NULL;
...>
}
--->8----

There are a lot more hits in compat/nedmallox and compat/regex but these
are borrowed code we rather do not want to maintain our own forks for,
and this patch refrains from touching them.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>