Fixes for unit tests discovered by NAG.

Three fixes were implemented to get these tests to pass in a build with
the NAG Fortran compiler and mpi-serial:

1) Nullify/deallocate pointers before each use, to avoid having them
   point to temporary storage that is too small.

   (There may be memory leaks here as well, but those are not addressed
   by this commit. It is more difficult to track these down because
   AttrVect uses pointers rather than allocatable arrays.)

2) Do not use unit 6 for the log file, since unit 6 is pre-connected to
   standard output. Also, check to make sure that the "open" statement
   succeeds before continuing with tests.

3) Use "mpif.h" instead of the mpi module, so that mpi-serial can be
   used out of the box.