Fix #281 std::bad_optional_access in RawMasterFile::roi() (#282)

Fixes #281

src/RawMasterFile.cpp

@@ -194,6 +194,15 @@ ScanParameters RawMasterFile::scan_parameters() const {
 }
 
 std::optional<ROI> RawMasterFile::roi() const {
+    if (!m_rois) {
+        return std::nullopt;
+    }
+
+    if (m_rois->empty()) {
+        throw std::runtime_error(LOCATION +
+                                 "Zero ROIs in metadata.");
+    }
+
     if (m_rois.value().size() > 1) {
         throw std::runtime_error(LOCATION +
                                  "Multiple ROIs present, use rois() method.");