Fixed null deref found by clang analyzer

Change-Id: I753af97065c413eaee8bdaf604f521cf6bb8933a

IO/Core/vtkInputStream.cxx

@@ -42,6 +42,7 @@ void vtkInputStream::StartReading()
   if(!this->Stream)
     {
     vtkErrorMacro("StartReading() called with no Stream set.");
+    return;
     }
   this->StreamStartPosition = this->Stream->tellg();
 }