Fix check for outdated cache entries in SensorCache::getAverage()

CollectAgent/sensorcache.cpp

@@ -67,8 +67,7 @@ int64_t SensorCache::getAverage(cacheEntry_t &entry, uint64_t avg) {
   TimeStamp ts;
 
   if (entry.size() > 0) {
-    //std::cout << ts.getRaw() - entry.back().timestamp << std::endl;
-    if (ts.getRaw() - entry.back().timestamp > avg * NS_PER_S) {
+    if (ts.getRaw() - entry.front().timestamp < avg * NS_PER_S) {
       throw std::out_of_range("Sid outdated");
       return 0;
     }