Fix a bug that 1969-12-31 23:59:59 UTC (Unix time: -1) causes an error
Fix a bug that 1969-12-31 23:59:59 UTC (Unix time: -1) causes an error

Redmine: fixes #2553

naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga