Fix an occasional segfault due to improper vector bounds in rng()

Thanks to forum user utunnels for spotting this (see http://www.cataclysmdda.com/smf/index.php?topic=1478).

Seems simple and obvious enough to commit directly, rather than via pull request. However, it's also low-priority (rare crash), and it seems that people have been a little confused by stuff not happening via pull request in the recent past (see discussion in #1174).

I'm submitting a pull request in part because I'd like feedback (by fellow maintainers OR the community at large) as to how cautious/by-the-book I should be in low-priority cases like this.