According to MPI-3.1 P.121, `ni` for `MPI_COMBINER_HINDEXED_BLOCK`
should be `2`, not `2 + count`.
This bug was introduced in 113b45b4 (when `MPI_Type_create_hindexed_block`
support is added in Open MPI) and fixed partially in 7f5314ee and 8de93982.
This commit fixes the remaining part.
Probably this bug has no user impact. It only consumes a bit more memory.