Revert "jffs2: Fix possible null-pointer dereferences in jffs2_add_frag_to_fragtree()"

This reverts commit f2538f99. The patch
stopped JFFS2 from being able to mount an existing filesystem with the
following errors:

 jffs2: error: (77) jffs2_build_inode_fragtree: Add node to tree failed -22
 jffs2: error: (77) jffs2_do_read_inode_internal: Failed to build final fragtree for inode #5377: error -22

Fixes: f2538f99 ("jffs2: Fix possible null-pointer dereferences...")
Cc: stable@vger.kernel.org
Suggested-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Richard Weinberger <richard@nod.at>