[BUG] tot_len_ratio being inf when src.size() is zero

Thanks for creating and maintaining this useful tool!

I'd like to report a bug in the following lines: `fast_align.cc#L261~L264`.
https://github.com/clab/fast_align/blob/7c2bbca3d5d61ba4b0f634f098c4fcf63c1373e1/src/fast_align.cc#L264
When `src.size()` is zero, `inf` is added to `*tot_len_ratio`. This, in turn, sets `mean_srclen_multiplier` to `inf` ([fast_align.cc#L291](https://github.com/clab/fast_align/blob/master/src/fast_align.cc#L291)), and subsequently `log_prob` to `inf`.

I think this can be avoided by adding a `continue;` statement when `src.size()` is zero.
Hope this helps!