UBIFS: fix error handling in dump_lpt_leb()

This patch checks the return value of 'ubifs_unpack_nnode()'.
If this function returns an error, 'nnode' may not be
initialized, so just print an error message and break.

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>