ext3: fix possible non-initialized variable on htree_dirblock_to_tree()

This is a backport of ext4 commit 90b0a973 which fixes a possible
non-initialized variable on htree_dirblock_to_tree().
Ext3 has the same non initialized variable, but, in any case it will be
initialized by ext3_get_blocks_handle(), which will avoid the bug to be
triggered, but, the non-initialized variable by htree_dirblock_to_tree() is
still a bug.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>