Fix SIGFPE on small number of overlaps

[jchorl]

Greetings,

I found that with a small number of overlaps, logger_step = overlaps.size() / RACON_LOGGER_BIN_SIZE will yield 0.

This causes a divide-by-zero:

                        log_bar_idx = window_idx / logger_step;

Now, there are in-fact overlaps, so the code should still execute. Let me know what you think of the fix. We could also just use an if below.