sstables: avoid copying val

Noted by Avi while we were searching for a fix for an unrelated bug.
We haven't actually seen this trigger, but the current in-tree behavior
is wrong.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>

sstables/sstables.cc

@@ -442,7 +442,7 @@ future<> parse(random_access_reader& in, disk_hash<Size, Key, Value>& h) {
 
 template <typename Key, typename Value>
 future<> write(file_writer& out, std::unordered_map<Key, Value>& map) {
-    return do_for_each(map.begin(), map.end(), [&out, &map] (auto val) {
+    return do_for_each(map.begin(), map.end(), [&out, &map] (auto& val) {
         Key key = val.first;
         Value value = val.second;
         return write(out, key, value);