btrfs: Don't pass NULL ptr to func that may deref it.

Hi,

In fs/btrfs/inode.c::fixup_tree_root_location() we have this code:

...
if (!path) {
err = -ENOMEM;
goto out;
}
...
out:
btrfs_free_path(path);
return err;

btrfs_free_path() passes its argument on to other functions and some of
them end up dereferencing the pointer.
In the code above that pointer is clearly NULL, so btrfs_free_path() will
eventually cause a NULL dereference.

There are many ways to cut this cake (fix the bug). The one I chose was to
make btrfs_free_path() deal gracefully with NULL pointers. If you
disagree, feel free to come up with an alternative patch.

Signed-off-by: Jesper Juhl <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/ctree.c

···
 /* this also releases the path */
 void btrfs_free_path(struct btrfs_path *p)
 {
+	if (!p)
+		return;
 	btrfs_release_path(NULL, p);
 	kmem_cache_free(btrfs_path_cachep, p);
 }