fixes #213 - handle GetColumnNames with empty column row (#215)

* fix GetColumnNames() undefined behavior on empty label row

GetColumnNames() did .begin() + (mRowNameIdx + 1) on the label row
without checking that the row has enough elements. When the label row
is empty (e.g. after InsertRow on an empty document with LabelParams(0,0)),
.begin() wraps a null pointer and adding to it is undefined behavior.

Add a bounds check before the iterator arithmetic so it returns an
empty vector instead of hitting UB.

* add test for get column labels of empty label row

* minor code style change, bump version

---------

Co-authored-by: MarkLee131 <kaixuan.li@ntu.edu.sg>

Author: d99kris

CMakeLists.txt

@@ -196,6 +196,7 @@ if(RAPIDCSV_BUILD_TESTS)
   add_unit_test(test104)
   add_unit_test(test105)
   add_unit_test(test106)
+  add_unit_test(test107)
 
   # perf tests
   add_perf_test(ptest001)

src/rapidcsv.h

@@ -2,7 +2,7 @@
  * rapidcsv.h
  *
  * URL:      https://github.com/d99kris/rapidcsv
- * Version:  8.95
+ * Version:  8.96
  *
  * Copyright (C) 2017-2026 Kristofer Berggren
  * All rights reserved.
@@ -1473,9 +1473,12 @@ namespace rapidcsv
     {
       if (mLabelParams.mColumnNameIdx >= 0)
       {
-        return std::vector<std::string>(mData.at(static_cast<size_t>(mLabelParams.mColumnNameIdx)).begin() +
-                                        (mLabelParams.mRowNameIdx + 1),
-                                        mData.at(static_cast<size_t>(mLabelParams.mColumnNameIdx)).end());
+        const std::vector<std::string>& labelRow = mData.at(static_cast<size_t>(mLabelParams.mColumnNameIdx));
+        const size_t offset = static_cast<size_t>(mLabelParams.mRowNameIdx + 1);
+        if (offset <= labelRow.size())
+        {
+          return std::vector<std::string>(labelRow.begin() + static_cast<int>(offset), labelRow.end());
+        }
       }
 
       return std::vector<std::string>();

tests/test107.cpp

@@ -0,0 +1,31 @@
+// test107.cpp - get column labels of empty label row
+
+#include <rapidcsv.h>
+#include "unittest.h"
+
+int main()
+{
+  int rv = 0;
+
+  try
+  {
+    // Empty document, specify column and row labels available
+    std::string csv = "";
+    std::istringstream s(csv);
+    rapidcsv::Document doc(s, rapidcsv::LabelParams(0, 0));
+
+    // InsertRow create a row with 0 columns
+    doc.InsertRow<std::string>(0);
+
+    // Get column names
+    std::vector<std::string> names = doc.GetColumnNames();
+    unittest::ExpectEqual(size_t, names.size(), size_t(0));
+  }
+  catch (const std::exception& ex)
+  {
+    std::cout << "exception: " << ex.what() << std::endl;
+    rv = 1;
+  }
+
+  return rv;
+}