Fix stack overflow caused by scanPeriod of 0s

[davidgyoung]

When upgrading an existing app to 2.12 the ScanState serialization file would not yet exist. Then when the app ran for the first time, it would find a missing file when deserializing it, yielding a scan period of 0. This would then cause a stackoverflow constantly stopping and starting scans. This was reported in #571.

The fix is to check if the scan period is zero and if so, immediately quit the scan job. A test showing the fix works is in the log below.

$ adb logcat | grep ScanJob
08-27 16:07:42.219  1125  1125 I ActivityManager: Start proc 15083:org.altbeacon.beaconreference/u0a236 for service org.altbeacon.beaconreference/org.altbeacon.beacon.service.ScanJob
08-27 16:07:42.631 15083 15083 I ScanJob : Running periodic scan job: instance is org.altbeacon.beacon.service.ScanJob@608a65e
08-27 16:07:42.638 15083 15083 D ScanJob : Processing 0 queued scan resuilts
08-27 16:07:42.638 15083 15083 D ScanJob : Done processing queued scan resuilts
08-27 16:07:42.638 15083 15083 I ScanJob : scanJob version 2.12.1-2-gd170b8c-SNAPSHOT is starting up on the main process
08-27 16:07:42.679 15083 15083 E ScanState: 	at org.altbeacon.beacon.service.ScanJob.restartScanning(ScanJob.java:150)
08-27 16:07:42.679 15083 15083 E ScanState: 	at org.altbeacon.beacon.service.ScanJob.startScanning(ScanJob.java:200)
08-27 16:07:42.679 15083 15083 E ScanState: 	at org.altbeacon.beacon.service.ScanJob.onStartJob(ScanJob.java:77)
08-27 16:07:42.806 15083 15083 W ScanJob : Starting scan with scan period of zero.  Exiting ScanJob.