Fixes #4548: Move r31467 to v1.8 branch (Fix a parameter check. Thanks)

---svn-pre-commit-ignore-below---

r31467 [[BR]]
Fix a parameter check. Thanks Lisandro Dalcin for the bug report.
More info @ http://www.open-mpi.org/community/lists/devel/2014/04/14561.php

cmr=v1.8.2:reviewer=ompi-rm1.8

r31491 [[BR]]
op_commutative.c: don't check if the op is predefined or not

MPI_OP_COMMUTATIVE should work on all MPI_Op's -- regardless of
whether they are predefined or not.

Refs #4548.

r31492 [[BR]]
op_commutative.c: ...but we should check for MPI_OP_NULL

Refs #4548

r31497 [[BR]]
op_commutative.c: use ompi_op_is_commute(), not the expanded test

One more commit for this ticket... as pointed out by Giles, we have
ompi_op_is_commute().  We should use that instead of replicating the
logic for the test.

Refs #4548