Fix iallgather problem with intercommunicators

A problem was found with the libnbc MPI_Iallgather
routine when using intercommunicators.  Special
thanks to Takahiro Kawashima(Fujitsu) for the patch
and a test case.  Verified master fails without the
patch and the test passes with the patch applied.

fixes #219
Merge pull request #18 from hppritcha/topic/fix-219-v1.8

Fix iallgather problem with intercommunicators