GathervVectors() does not work when data is null everywhere

Created by: pnorbert

source/adios2/helper/adiosMPIFunctions.inl: template void GathervVectors(const std::vector &in, std::vector &out, size_t &position, MPI_Comm mpiComm, const int rankDestination)

rank 0 returns without entering MPI_Gatherv() if the total size to be collected is 0, leaving the rest of processes hanging in the call.

The bug only presents itself on >128 processes on Titan. Any example without attributes would show the bug.