portals4: remove pointer update from unmatched improbe

Previously, the message pointer in an improbe call in the portals4
netmod layer was set to MPI_MESSAGE_NULL if there was no match. This
is incorrect because the ch3 layer eventually returns either a valid
pointer or NULL. As ch3 already starts the value at NULL, we can just
omit the update.
Signed-off-by: Pavan Balaji <balaji@anl.gov>

src/mpid/ch3/channels/nemesis/netmod/portals4/ptl_probe.c

@@ -241,7 +241,6 @@ int MPID_nem_ptl_improbe(MPIDI_VC_t *vc, int source, int tag, MPID_Comm *comm, i
     }
     else {
         MPID_Request_release(req);
-        *message = MPI_MESSAGE_NULL;
     }
 
  fn_exit: