BergerRigoutsos gets incorrect MPI_TAG_UB

[humhang]

in SAMRAI/mesh/BergerRigoutsos.cpp, BergerRigoutsos::setupMPIDependentData(), one should check the value of flag instead of the returned attribute value pointer tag_upper_bound_ptr to determine whether MPI_Attr_get succeeds.

This should be a typo. If it fails, the flag is set to false but the attribute pointer can be anything, not necessarily being null, per MPI standard.