Fix offset assertion, for empty procs first tree is allowed to equal num trees

[lukasdreyer]

Describe your changes here:

This PR fixes the following mwe, that runs into the wrong assertion on 8 procs.
On empty procs, first_tree might equal num_trees.

#include <t8.h>
#include <t8_cmesh/t8_cmesh_examples.h>
#include <t8_schemes/t8_default/t8_default_cxx.hxx>
#include <t8_forest/t8_forest_general.h>

static void
t8_mwe_cmesh_partition (){
  int level = 1;
  t8_cmesh_t cmesh = t8_cmesh_new_hypercube(T8_ECLASS_LINE, sc_MPI_COMM_WORLD, 0, 0, 0);
  t8_cmesh_t partitioned_cmesh;
  t8_cmesh_init(&partitioned_cmesh);
  t8_scheme_cxx_t *ts = t8_scheme_new_default_cxx();
  t8_scheme_cxx_ref(ts);
  t8_cmesh_set_derive(partitioned_cmesh, cmesh);
  t8_cmesh_set_partition_uniform(partitioned_cmesh, level, ts);
  t8_cmesh_commit(partitioned_cmesh, sc_MPI_COMM_WORLD);
  
  t8_forest_t forest = t8_forest_new_uniform(partitioned_cmesh, ts, level, 0, sc_MPI_COMM_WORLD);
  t8_forest_unref (&forest);
}

int
main (int argc, char **argv)
{
  int mpiret;

  mpiret = sc_MPI_Init (&argc, &argv);
  SC_CHECK_MPI (mpiret);

  sc_init (sc_MPI_COMM_WORLD, 1, 1, NULL, SC_LP_ESSENTIAL);
  t8_init (SC_LP_DEBUG);

  t8_mwe_cmesh_partition();

  sc_finalize ();

  mpiret = sc_MPI_Finalize ();
  SC_CHECK_MPI (mpiret);
  return 0;
}

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[lukasdreyer]

The wrong assertion also prevents some of our tests to succeed on multiple procs.

Check this by changing the following lines in sc_mpi.m4:

  if test "x$$1_MPIRUN" = xmpirun ; then
    $1_MPI_TEST_FLAGS="-np 2"
  elif test "x$$1_MPIRUN" = xmpiexec ; then
    $1_MPI_TEST_FLAGS="-n 2"
  else
    $1_MPIRUN=
  fi