Fix git-fsck-objects SIGSEGV/divide-by-zero

If you try to fsck a repository that isn't entirely empty, but that has no 
inter-object references (ie all the objects are blobs, and don't refer to 
anything else), git-fsck-objects currently fails.

This probably cannot happen in practice, but can be tested with something 
like

	git init-db
	touch dummy
	git add dummy
	git fsck-objects

where the fsck will die by a divide-by-zero when it tries to look up the 
references from the one object it found (hash_obj() will do a modulus by 
refs_hash_size).

On some other archiectures (ppc, sparc) the divide-by-zero will go 
unnoticed, and we'll instead SIGSEGV when we hit the "refs_hash[j]" 
access.

So move the test that should protect against this from mark_reachable() 
into lookup_object_refs(), which incidentally in the process also fixes 
mark_reachable() itself (it used to not mark the one object that _was_ 
reachable, because it decided that it had no refs too early).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

diff --git a/object-refs.c b/object-refs.c
index b0034e4..98ea100 100644
--- a/object-refs.c
+++ b/object-refs.c

@@ -55,9 +55,13 @@ static void add_object_refs(struct objec
 
 struct object_refs *lookup_object_refs(struct object *obj)
 {
-	int j = hash_obj(obj, refs_hash_size);
 	struct object_refs *ref;
+	int j;
 
+	/* nothing to lookup */
+	if (!refs_hash_size)
+		return NULL;
+	j = hash_obj(obj, refs_hash_size);
 	while ((ref = refs_hash[j]) != NULL) {
 		if (ref->base == obj)
 			break;

@@ -125,9 +129,6 @@ void mark_reachable(struct object *obj, 
 
 	if (!track_object_refs)
 		die("cannot do reachability with object refs turned off");
-	/* nothing to lookup */
-	if (!refs_hash_size)
-		return;
 	/* If we've been here already, don't bother */
 	if (obj->flags & mask)
 		return;