Fix null pointers passed to memcmp/memcpy in skip_list

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

SkipList.cpp

@@ -38,6 +38,9 @@ std::span<const uint8_t> keyAfter(Arena &arena, std::span<const uint8_t> key) {
 
 std::span<const uint8_t> copyToArena(Arena &arena,
                                      std::span<const uint8_t> key) {
+  if (key.size() == 0) {
+    return std::span<const uint8_t>();
+  }
   auto result = std::span<uint8_t>(new (arena) uint8_t[key.size()], key.size());
   memcpy(result.data(), key.data(), key.size());
   return result;
@@ -97,7 +100,7 @@ force_inline bool getCharacter(const KeyInfo &ki, int character,
 
 bool operator<(const KeyInfo &lhs, const KeyInfo &rhs) {
   int i = std::min(lhs.key.size(), rhs.key.size());
-  int c = memcmp(lhs.key.data(), rhs.key.data(), i);
+  int c = i > 0 ? memcmp(lhs.key.data(), rhs.key.data(), i) : 0;
   if (c != 0)
     return c < 0;
 
@@ -287,7 +290,8 @@ private:
 
   static force_inline bool less(const uint8_t *a, int aLen, const uint8_t *b,
                                 int bLen) {
-    int c = memcmp(a, b, std::min(aLen, bLen));
+    int n = std::min(aLen, bLen);
+    int c = n > 0 ? memcmp(a, b, n) : 0;
     if (c < 0)
       return true;
     if (c > 0)
@@ -754,7 +758,8 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
 
   void addWrites(const ConflictSet::WriteRange *writes, int count,
                  int64_t writeVersion) {
-    auto points = std::vector<KeyInfo>(count * 2);
+    auto points = std::vector<KeyInfo>();
+    points.reserve(count * 2);
     Arena arena;
 
     for (int r = 0; r < count; r++) {