[FlashR]: can't load a dense matrix from a directory

[Da Zheng]

> data <- fm.load.dense.matrix("/mnt/nfs/zhengda/data/Criteo", in.mem=FALSE, name="Criteo-all.mat")
R: /mnt/nfs2/zhengda/FlashX-stable/matrix/data_io.cpp:346: void fm::read_complete(int, char*, size_t, size_t): Assertion `ret >= 0' failed.
Aborted