ext2: check err when partial != NULL

Check err when partial == NULL is meaningless because
partial == NULL means getting branch successfully without
error.

CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20191105045100.7104-1-cgxu519@mykernel.net


Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Jan Kara <jack@suse.cz>