Fix typo.
coll/basic: fix segmentation fault in neighborhood collectives if the degree
of the topology is higher than the communicator size

It is possible to have a topology degree higher than the size of the communicator.
For example, a periodic cartesian communicator on MPI_COMM_SELF. This will leave
the neighborhood collectives with a request buffer that is too small. This commit
adds a call that will dynamically increase the size of the request buffer if it
is too small.

A better fix would be to create the topology *before* calling the coll_select
routine on a communicator. This will take some discussion and the solution will
not likely be ready anytime soon.

Thanks to Lisandro Dalcin for reporting this.

Original thread: http://www.open-mpi.org/community/lists/devel/2014/08/15713.php

cmr=v1.8.3:reviewer=jsquyres