adding check for timestamps which don't belong to a job

analytics/operators/persystsql/PerSystSqlOperator.cpp

@@ -175,10 +175,14 @@ void PerSystSqlOperator::compute(U_Ptr unit, qeJobData& jobData) {
 		return;
 	} else {
 		measurement_ts = _buffer[0].timestamp;
+		if(measurement_ts < jobData.startTime){
+			LOG(debug) << "PerSystSQL Operator: timestamps not part of job.";
+			return; 
+		}
 	}
 	Aggregate_info_t agg_info;
 	agg_info.timestamp = (measurement_ts/1e9);
-	compute_internal(unit, _buffer, agg_info, measurement_ts); //it can't possibly be zero here
+	compute_internal(unit, _buffer, agg_info, measurement_ts);
 
 	if( _backend == MARIADB ) {
 		if (!_persystdb->isInitialized()