sheep/ops: fix potential type conversion error

Similar to lib/logger, epoch_log_read_with_timestamp will return -1
for any failure and it cannot be assigned to a variable in type of
size_t. Otherwise, the error branch will never be reached.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>