fix a bug in accessing SSDs.

[Da Zheng]

If we store a block matrix to SSDs, we may get this error.
This bug has been temporarily fixed by converting the block matrix to a normal matrix. We need to handle it more gracefully.

> emdata <- fm.conv.store(data, in.mem = FALSE)
mkdir: Permission denied

 *** caught segfault ***
address (nil), cause 'memory not mapped'

Traceback:
 1: .Call("R_FM_conv_store", fm, as.logical(in.mem), as.character(name),     PACKAGE = "FlashR")
 2: fm.conv.store(data, in.mem = FALSE)

Possible actions:
1: abort (with core dump, if enabled)
2: normal R exit
3: exit R without saving workspace
4: exit R saving workspace