work around occasional segfault from accessing nonexistent last unigram

The last call to this code block is for the probability of word unigram_count, i.e., the n+1th of n unigrams. In some cases this causes a segfault, presumably related to whatever is happening with mmap when the FindBlanks is initialized. I thought the right thing to do would be to just break one iter sooner but that caused other issues and I didn't have time to debug. This workaround is quite a kludge but avoids the issue.

I'm posting it as a request with the implicit request that you'll see what I'm getting at here and know much more quickly than me if this is a problem. I should point out that I didn't test with closed-class arpas; could be this is for those cases (and you reserve id 0 for unk so in that case you really do go from 1 to n?).

Sorry this is so slapdash; I hope to be able to isolate specifically why I get the segfault serializing some arpas but not all but I also would love it if this rang some bell for you.