[bugfix] Invalid pointer comparisons.

The code was performing a null pointer check
using (void*)Data <= 0 instead of Data == NULL 
(or nullptr). This results in a compilation
error with clang trunk (future clang 4.0).

ThirdParty/xdmf2/vtkxdmf2/libsrc/XdmfDsmComm.cxx

@@ -52,7 +52,7 @@ XdmfDsmComm::Receive(XdmfDsmMsg *Msg){
         XdmfErrorMessage("Cannot Receive Message of Length = " << Msg->Length);
         return(XDMF_FAIL);
     }
-    if(Msg->Data <= 0 ){
+    if(Msg->Data == NULL){
         XdmfErrorMessage("Cannot Receive Message into Data Buffer = " << Msg->Length);
         return(XDMF_FAIL);
     }
@@ -66,7 +66,7 @@ XdmfDsmComm::Send(XdmfDsmMsg *Msg){
         XdmfErrorMessage("Cannot Send Message of Length = " << Msg->Length);
         return(XDMF_FAIL);
     }
-    if(Msg->Data <= 0 ){
+    if(Msg->Data == NULL) {
         XdmfErrorMessage("Cannot Send Message from Data Buffer = " << Msg->Length);
         return(XDMF_FAIL);
     }