In addWrites, the sizing constructor std::vector<KeyInfo>(count * 2)
was used instead of reserve(count * 2), leaving count*2 default-
constructed (null/empty key) entries that were then compared via
operator<, passing nullptr to memcmp. Switch to reserve so only real
entries exist.
Additionally guard the memcmp/memcpy calls in operator<, SkipList::less,
and copyToArena against empty spans (where data() may be nullptr), which
is reachable on the first setOldestVersion with an empty removal key.
Closes#64
weaselbot