Update SU2_CFD/src/output/COutput.cpp

Co-authored-by: Pedro Gomes <38071223+pcarruscag@users.noreply.github.com>

Author: bigfooted

SU2_CFD/src/output/COutput.cpp

@@ -1565,7 +1565,7 @@ void COutput::PreprocessVolumeOutput(CConfig *config){
 
       VolumeOutputField &Field = volumeOutput_Map.at(fieldReference);
       /*--- Loop through all fields specified in the config ---*/
-      for (size_t iReqField = 0; iReqField < nRequiredVolumeFields; iReqField++) {
+      for (size_t iReqField = 0; iReqField < requiredVolumeFields.size(); iReqField++) {
 
         RequiredField = requiredVolumeFields[iReqField];
         if (((RequiredField == Field.outputGroup) || (RequiredField == fieldReference)) && (Field.offset == -1)) {