Potential NULL pointer dereference : forgetting to check the return value of libxml2 API xmlDocGetRootElement

This issue lies in the IO/Infovis/vtkXMLTreeReader.cxx:219.

According to the libxml2 API document, the API xmlDocGetRootElement can Returns: the #xmlNodePtr for the root or NULL. However, the fucntion 'vtkXMLTreeReaderProcessElement' try to dereference its return value (lies in vtkXMLTreeReader.cxx:92) without checking it at first. This may cause application to crash.