Attempt to fix crash when ScanJob starts scanning right after it stop…

…s it in a thread race condition.

lib/src/main/java/org/altbeacon/beacon/service/ScanJob.java

@@ -54,6 +54,7 @@ public class ScanJob extends JobService {
     @Nullable
     private ScanHelper mScanHelper;
     private boolean mInitialized = false;
+    private boolean mStopCalled = false;
 
     @Override
     public boolean onStartJob(final JobParameters jobParameters) {
@@ -87,6 +88,12 @@ public void run() {
                 }
                 LogManager.d(TAG, "Done processing queued scan resuilts");
 
+                if (mStopCalled) {
+                    LogManager.d(TAG, "Quitting scan job before we even start.  Somebody told us to stop.");
+                    ScanJob.this.jobFinished(jobParameters , false);
+                    return;
+                }
+
                 boolean startedScan;
                 if (mInitialized) {
                     LogManager.d(TAG, "Scanning already started.  Resetting for current parameters");
@@ -176,6 +183,7 @@ private void startPassiveScanIfNeeded() {
 
     @Override
     public boolean onStopJob(JobParameters params) {
+        mStopCalled = true;
         if (params.getJobId() == getPeriodicScanJobId(this)) {
             LogManager.i(TAG, "onStopJob called for periodic scan " + this);
         }