LSTMTrainer: Catch empty vectors

The new test in LSTMTrainer::UpdateErrorGraph fixes an assertion
(see issues #644, #792).

The new test in LSTMTrainer::ReadTrainingDump was added to improve
the robustness of the code.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

lstm/lstmtrainer.cpp

@@ -918,6 +918,10 @@ bool LSTMTrainer::SaveTrainingDump(SerializeAmount serialize_amount,
 // Reads previously saved trainer from memory.
 bool LSTMTrainer::ReadTrainingDump(const GenericVector<char>& data,
                                    LSTMTrainer* trainer) {
+  if (data.size() == 0) {
+    tprintf("Warning: data size is zero in LSTMTrainer::ReadTrainingDump\n");
+    return false;
+  }
   return trainer->ReadSizedTrainingDump(&data[0], data.size());
 }
 
@@ -1298,8 +1302,9 @@ STRING LSTMTrainer::UpdateErrorGraph(int iteration, double error_rate,
   if (error_rate < best_error_rate_) {
     // This is a new (global) minimum.
     if (tester != NULL) {
-      result = tester->Run(worst_iteration_, worst_error_rates_,
-                           worst_model_data_, CurrentTrainingStage());
+      if (worst_model_data_.size() != 0)
+        result = tester->Run(worst_iteration_, worst_error_rates_,
+                             worst_model_data_, CurrentTrainingStage());
       worst_model_data_.truncate(0);
       best_model_data_ = model_data;
     }