C assert error on encrypted home of Ubuntu

[Takashi Kato]

Ubuntu can encrypt home directory however this restricts path length 128 or so. If a cache file name is longer than this, Sagittarius stops with the following error:

ASSERT failure /home/takashi/.scheme-env/work/sagittarius-0.8.9/src/os/posix/file.c:174: posix_is_open(self)

It would be convenient if this simply disables cache.

[Takashi Kato]

Adding file open check on cache writing. Fixes #235

→ <<cset e67e6bfbfecd>>

[Takashi Kato]

It's not fixed

[Takashi Kato]

Adding open check after cache read/write. Should fix #235

→ <<cset 6d9d32d84f72>>