sheep/store: add epoch file length checking

If epoch file length is shorter than size of time due to file broken,
epoch_stat.st_size - sizeof(*timestamp) is negative. However,
the third parameter of function xread will get it as type size_t,
then xread will think there are many data to be read. Therefore,
a file length checking to prevent this situation is needed.

v2 does nothing changed logically but uses a variable to store
the value of the express 'epoch_stat.st_size - sizeof(*timestamp)'

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>