Fix a bogus warning about a null pointer in a lambda.

Fix a bogus warning about a null pointer in a lambda.

I get the following warning:

/home/drwells/Documents/Code/CPP/dealii-dev/source/base/mpi.cc: In static member function 'static constexpr void dealii::Utilities::MPI::create_mpi_data_type_n_bytes(std::size_t)::<lambda(ompi_datatype_t**)>::_FUN(ompi_datatype_t**)':
/home/drwells/Documents/Code/CPP/dealii-dev/source/base/mpi.cc:384:15: warning: 'this' pointer is null [-Wnonnull]
  384 |               }};
      |               ^
/home/drwells/Documents/Code/CPP/dealii-dev/source/base/mpi.cc:376:15: note: in a call to non-static member function 'dealii::Utilities::MPI::create_mpi_data_type_n_bytes(std::size_t)::<lambda(ompi_datatype_t**)>'
  376 |               [](MPI_Datatype *p) {
      |               ^

This isn't a problem since deleters don't store any kind of state but we can
work around it by splitting the constructor call.