Revert some of the earlier change

dlib/dnn/trainer.h

@@ -1068,18 +1068,15 @@ namespace dlib
                     return true;
             }
 
-            // if we haven't seen much data yet then just say false.
-            if (gradient_updates_since_last_sync < 30)
+            // if we haven't seen much data yet then just say false.  Or, alternatively, if
+            // it's been too long since the last sync then don't reload either.
+            if (gradient_updates_since_last_sync < 30 || previous_loss_values.size() < 2*gradient_updates_since_last_sync)
                 return false;
 
             // Now look at the data since a little before the last disk sync.  We will
             // check if the loss is getting better or worse.
             running_gradient g;
-            const size_t first_index
-                = previous_loss_values.size() < 2 * gradient_updates_since_last_sync
-                ? 0
-                : previous_loss_values.size() - 2 * gradient_updates_since_last_sync;
-            for (size_t i = first_index; i < previous_loss_values.size(); ++i)
+            for (size_t i = previous_loss_values.size() - 2*gradient_updates_since_last_sync; i < previous_loss_values.size(); ++i)
                 g.add(previous_loss_values[i]);
 
             // if the loss is very likely to be increasing then return true