db_iter.cc: DBIter::Next(): minor improve (#7407)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7407

Reviewed By: ajkr

Differential Revision: D23817122

Pulled By: jay-zhuang

fbshipit-source-id: 62bf43e4d780fad8c682edd750b4800b5b8f4a77

db/db_iter.cc

@@ -147,13 +147,13 @@ void DBIter::Next() {
 
   local_stats_.next_count_++;
   if (ok && iter_.Valid()) {
-    Slice prefix;
     if (prefix_same_as_start_) {
       assert(prefix_extractor_ != nullptr);
-      prefix = prefix_.GetUserKey();
+      const Slice prefix = prefix_.GetUserKey();
+      FindNextUserEntry(true /* skipping the current user key */, &prefix);
+    } else {
+      FindNextUserEntry(true /* skipping the current user key */, nullptr);
     }
-    FindNextUserEntry(true /* skipping the current user key */,
-                      prefix_same_as_start_ ? &prefix : nullptr);
   } else {
     is_key_seqnum_zero_ = false;
     valid_ = false;