jfjoch_viewer: Trace more potential memory issues

reader/JFJochHttpReader.cpp

@@ -196,6 +196,10 @@ bool JFJochHttpReader::LoadImage_i(std::shared_ptr<JFJochReaderDataset> &dataset
     if (!res || res->status != httplib::StatusCode::OK_200)
         return false;
 
+    if (res->body.empty()) {
+        return false;
+    }
+
     try {
         buffer.resize(res->body.size());
         memcpy(buffer.data(), res->body.data(), res->body.size());

viewer/JFJochImageReadingWorker.cpp

@@ -209,6 +209,11 @@ void JFJochImageReadingWorker::UpdateDataset_i(const std::optional<DiffractionEx
             file_reader.UpdateGeomMetadata(experiment.value());
         dataset = file_reader.GetDataset();
     }
+    if (!dataset) {
+        logger.Error("UpdateDataset_i: dataset is null (http_mode={}) - skipping update to avoid crash", http_mode);
+        return;
+    }
+
     curr_experiment = dataset->experiment;
     curr_experiment.ImportIndexingSettings(indexing_settings);
     curr_experiment.ImportAzimuthalIntegrationSettings(azint_settings);
@@ -341,6 +346,12 @@ void JFJochImageReadingWorker::UpdateUserMask_i(const std::vector<uint32_t> &mas
         file_reader.UpdateUserMask(mask);
         dataset = file_reader.GetDataset();
     }
+
+    if (!dataset) {
+        logger.Error("UpdateUserMask_i: dataset is null (http_mode={}) - skipping update to avoid crash", http_mode);
+        return;
+    }
+
     UpdateAzint_i(dataset.get());
 
     emit datasetLoaded(dataset);

viewer/JFJochViewerDatasetInfo.cpp

@@ -133,6 +133,9 @@ void JFJochViewerDatasetInfo::imageSelectedInChart(int64_t number) {
 
 void JFJochViewerDatasetInfo::UpdatePlot() {
     int index = combo_box->currentIndex();
+    if (combo_box->count() == 0 || index < 0)
+        return;
+
     int val = combo_box->itemData(index).toInt();
 
     if (dataset) {