Ensure comparison is of correct types

Refs #15623

Framework/DataHandling/src/SaveAscii2.cpp

@@ -439,7 +439,7 @@ void SaveAscii2::populateQMetaData() {
  */
 void SaveAscii2::populateSpectrumNumberMetaData() {
   std::vector<std::string> spectrumNumbers;
-  const auto nHist = m_ws->getNumberHistograms();
+  const size_t nHist = m_ws->getNumberHistograms();
   for (size_t i = 0; i < nHist; i++) {
     const auto specNum = m_ws->getSpectrum(i)->getSpectrumNo();
     const auto specNumStr = boost::lexical_cast<std::string>(specNum);
@@ -453,7 +453,7 @@ void SaveAscii2::populateSpectrumNumberMetaData() {
  */
 void SaveAscii2::populateAngleMetaData() {
   std::vector<std::string> angles;
-  const auto nHist = m_ws->getNumberHistograms();
+  const size_t nHist = m_ws->getNumberHistograms();
   for (size_t i = 0; i < nHist; i++) {
     const auto specNo = m_ws->getSpectrum(i)->getSpectrumNo();
     const auto workspaceIndex = m_specToIndexMap[specNo];