arithmetic overflow in word-aligner/da.h

[froiss]

In functions ComputeZ and ComputeDLogZ, line 33 and 50:

const unsigned num_top = n - floor;

floor may be greater than n, which makes num_top reach values close to 2^32.

It happens in real life when the following conditions are met:

• fast_align is called with the flags --favor_diagonal and --optimize_tension, and
• the parallel corpus contains source sentences that are longer than their matching target sentence.

In that situation, fast_align.cc's main sometimes invokes ComputeDLogZ with i > n. ComputeDLogZ then calls ComputeZ with i > m (which triggers an assert error if the asserts are commented out).

Note: This is obviously related to those two commits
clab/fast_align@5fe669e
clab/fast_align@adfadde
from the clab/fast_align repo, although I am not quite sure why the second reverted the first.