bcachefs: Fix a memcpy call

Not supposed to pass a null ptr to memcpy (even if the size is 0).
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fsck.c

@@ -1161,7 +1161,8 @@ static int add_nlink(struct nlink_table *t, u64 inum, u32 snapshot)
 			return -ENOMEM;
 		}
 
-		memcpy(d, t->d, t->size * sizeof(t->d[0]));
+		if (t->d)
+			memcpy(d, t->d, t->size * sizeof(t->d[0]));
 		kvfree(t->d);
 
 		t->d = d;