Fix invalid reference in MultiGet due to vector resizing (#10702)

Summary:
Fix invalid reference in MultiGet due to resizing of the ```batches``` autovector.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10702

Test Plan: Run asan crash test

Reviewed By: riversand963

Differential Revision: D39608753

Pulled By: anand1976

fbshipit-source-id: 7a9e7fc6f436f08eb22003d0e6b0e1e4dcdc1a2a

db/version_set.cc

@@ -2776,6 +2776,11 @@ Status Version::MultiGetAsync(
   to_process.emplace_back(0);
 
   while (!to_process.empty()) {
+    // As we process a batch, it may get split into two. So reserve space for
+    // an additional batch in the autovector in order to prevent later moves
+    // of elements in ProcessBatch().
+    batches.reserve(batches.size() + 1);
+
     size_t idx = to_process.front();
     FilePickerMultiGet* batch = &batches.at(idx);
     unsigned int num_tasks_queued = 0;