staging: erofs: unzip_vle.c: Replace comparison to NULL.

Replace equal to NULL with logic unary operator,
and removing not equal to NULL comparison.

Signed-off-by: Cristian Sicilia <sicilia.cristian@gmail.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>