In the function ao_foreach_extent_file() an array concurrency is used to
store the concurrent levels, its size was AOTupleId_MaxSegmentFileNum,
which equals to (MAX_AOREL_CONCURRENCY - 1).  However the possible index
range is [0, MAX_AOREL_CONCURRENCY - 1], it exceeds the original size of
the array, so an array overflow can happen at runtime.

Fixed by correcting the array size, also define the size with
MAX_AOREL_CONCURRENCY instead of AOTupleId_MaxSegmentFileNum.
Reviewed-by: NZhenghua Lyu <zlv@pivotal.io>
Reviewed-by: NNing Yu <nyu@pivotal.io>