Fix NullPointerException: the JNI method __store_get may fail and return NULL

The mesos JAVA API will call driver->abort() if any scheduler callback throws
any exception, this will bring chronos into *deactivated* state and will not
receive any offer from mesos, so as can not start any job (task exactly) any
longer.

The exception ever happened is NullPointerException throwed by persistData().

Bug: https://github.com/airbnb/chronos/issues/263

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>