Fix for possible null DcmObject

Didn't happen with older dcmtk (3.6), but appears to happen
with newer versions so check for it.

Libs/DICOM/Core/ctkDICOMDatabase.cpp

@@ -468,12 +468,15 @@ void ctkDICOMDatabase::loadFileHeader (QString fileName)
       while (dataset->nextObject(stack, true) == EC_Normal)
         {
           DcmObject *dO = stack.top();
-          QString tag = QString("%1,%2").arg(
-                dO->getGTag(),4,16,QLatin1Char('0')).arg(
-                dO->getETag(),4,16,QLatin1Char('0'));
-          std::ostringstream s;
-          dO->print(s);
-          d->LoadedHeader[tag] = QString(s.str().c_str());
+          if (dO)
+            {
+              QString tag = QString("%1,%2").arg(
+                    dO->getGTag(),4,16,QLatin1Char('0')).arg(
+                    dO->getETag(),4,16,QLatin1Char('0'));
+              std::ostringstream s;
+              dO->print(s);
+              d->LoadedHeader[tag] = QString(s.str().c_str());
+            }
         }
     }
   return;