Fix heap use after free error in FilePrefetchBuffer (facebook#12211)

Summary:
Fix heap use after free error in FilePrefetchBuffer
Fix heap use after free error in FilePrefetchBuffer

Pull Request resolved: facebook#12211

Test Plan:
Ran db_stress in ASAN mode
```
==652957==ERROR: AddressSanitizer: heap-use-after-free on address 0x6150006d8578 at pc 0x7f91f74ae85b bp 0x7f91c25f90c0 sp 0x7f91c25f90b8
READ of size 8 at 0x6150006d8578 thread T48
    #0 0x7f91f74ae85a in void __gnu_cxx::new_allocator<rocksdb::BufferInfo*>::construct<rocksdb::BufferInfo*, rocksdb::BufferInfo*&>(rocksdb::BufferInfo**, rocksdb::BufferInfo*&) /mnt/gvfs/third-party2/libgcc/c00dcc6a3e4125c7e8b248e9a79c14b78ac9e0ca/11.x/platform010/5684a5a/include/c++/trunk/ext/new_allocator.h:163
    facebook#1 0x7f91f74ae85a in void std::allocator_traits<std::allocator<rocksdb::BufferInfo*> >::construct<rocksdb::BufferInfo*, rocksdb::BufferInfo*&>(std::allocator<rocksdb::BufferInfo*>&, rocksdb::BufferInfo**, rocksdb::BufferInfo*&) /mnt/gvfs/third-party2/libgcc/c00dcc6a3e4125c7e8b248e9a79c14b78ac9e0ca/11.x/platform010/5684a5a/include/c++/trunk/bits/alloc_traits.h:512
    facebook#2 0x7f91f74ae85a in rocksdb::BufferInfo*& std::deque<rocksdb::BufferInfo*, std::allocator<rocksdb::BufferInfo*> >::emplace_back<rocksdb::BufferInfo*&>(rocksdb::BufferInfo*&) /mnt/gvfs/third-party2/libgcc/c00dcc6a3e4125c7e8b248e9a79c14b78ac9e0ca/11.x/platform010/5684a5a/include/c++/trunk/bits/deque.tcc:170
    facebook#3 0x7f91f74b93d8 in rocksdb::FilePrefetchBuffer::FreeAllBuffers() file/file_prefetch_buffer.h:557
```

Reviewed By: ajkr

Differential Revision: D52575217

Pulled By: akankshamahajan15

fbshipit-source-id: 6811ec10a393f5a62fedaff0fab5fd6e823c2687

file/file_prefetch_buffer.h

@@ -551,7 +551,8 @@ class FilePrefetchBuffer {
   }
 
   void FreeAllBuffers() {
-    for (auto& buf : bufs_) {
+    while (!bufs_.empty()) {
+      BufferInfo* buf = bufs_.front();
       buf->ClearBuffer();
       bufs_.pop_front();
       free_bufs_.emplace_back(buf);