BP3 vs BP4 behavior changes.

Created by: NAThompson

Describe the bug

If we uncomment the bpIO.SetEngine("bp3") line, the following program aborts with an exception:

    adios2::ADIOS adios(adios2::DebugON);
    adios2::IO bpIO = adios.DeclareIO("BPFile_N2N");
    //bpIO.SetEngine("bp3");
    adios2::Variable<Real> random_data = bpIO.DefineVariable<Real>("random_data", {}, {}, {v.size()}, adios2::ConstantDims);
    std::string filename = "random_vector_" + std::to_string(state.range(0)) + ".bp";
    adios2::Engine bpWriter = bpIO.Open(filename, adios2::Mode::Write);
    bpWriter.Put<Real>(random_data, v.data());
    bpWriter.Close();


    adios2::Engine bpReader = bpIO.Open(filename, adios2::Mode::Read);

Desktop (please complete the following information):

• OS/Platform: Ubuntu 18.0.4
• Build: gcc 7.4.0, cmake 15

Additional context

This is found in ADIOS2Benchmarks