Sanitize input in DB::MultiGet() API (#6054)

Summary:
The new DB::MultiGet() doesn't validate input for num_keys > 1 and GCC-9 complains about it. Fix it by directly return when num_keys == 0
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6054

Test Plan: Build with GCC-9 and see it passes.

Differential Revision: D18608958

fbshipit-source-id: 1c279aff3c7fe6e9d5a6d085ed02550ecea4fdb2

db/db_impl/db_impl.cc

@@ -1881,6 +1881,9 @@ void DBImpl::MultiGet(const ReadOptions& read_options, const size_t num_keys,
                       ColumnFamilyHandle** column_families, const Slice* keys,
                       PinnableSlice* values, Status* statuses,
                       const bool sorted_input) {
+  if (num_keys == 0) {
+    return;
+  }
   autovector<KeyContext, MultiGetContext::MAX_BATCH_SIZE> key_context;
   autovector<KeyContext*, MultiGetContext::MAX_BATCH_SIZE> sorted_keys;
   sorted_keys.resize(num_keys);