SIGSEGV when using direct_reads

Hello there,
I am not sure if I use it wrong but when I turn on the RocksDB use_direct_reads, this SIGSEGV bug will show up when open the db. I used version 5.9.2. Hope someone can shed some light on this. Thanks!

### Expected behavior
The RocksDB direct I/O mode work fine.

### Actual behavior
This SIGSEGV appears when open db.

### Steps to reproduce the behavior
1. set use_direct_reads = true in options.h file.
2. invoke DB::Open api

### coredump
```
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007fa23a22d810 in _IO_sgetn_internal () from /lib64/libc.so.6
[Current thread is 1 (Thread 0x7fa23b44aa80 (LWP 38038))]
(gdb) bt
#0 0x00007fa23a22d810 in _IO_sgetn_internal () from /lib64/libc.so.6
#1 0x00007fa23a22ac0e in fread_unlocked () from /lib64/libc.so.6
#2 0x000000000074b25a in rocksdb::PosixSequentialFile::Read (this=0x28e05a0, n=8192, result=0x7ffefb636f30, scratch=0x2864000 "") at env/io_posix.cc:159
#3 0x000000000073f500 in rocksdb::ReadFileToString (env=, fname=..., data=data@entry=0x7ffefb637060) at env/env.cc:297
#4 0x00000000007198c5 in rocksdb::VersionSet::Recover (this=0x26ca2c0, column_families=..., read_only=read_only@entry=false) at db/version_set.cc:2817
#5 0x0000000000689018 in rocksdb::DBImpl::Recover (this=this@entry=0x28ce000, column_families=..., read_only=read_only@entry=false, error_if_log_file_exist=error_if_log_file_exist@entry=false,
error_if_data_exists_in_logs=error_if_data_exists_in_logs@entry=false) at db/db_impl_open.cc:361
```
### rocksdb log
```
2019/08/23-13:48:05.293022 7fa23b44aa80 RocksDB version: 5.9.2
2019/08/23-13:48:05.293072 7fa23b44aa80 Git sha rocksdb_build_git_sha:231fdc1546dd492e6cc567844acee512c40ef83f
2019/08/23-13:48:05.293077 7fa23b44aa80 Compile date Aug 23 2019
2019/08/23-13:48:05.293080 7fa23b44aa80 DB SUMMARY
2019/08/23-13:48:05.293260 7fa23b44aa80 CURRENT file: CURRENT
2019/08/23-13:48:05.293264 7fa23b44aa80 IDENTITY file: IDENTITY
2019/08/23-13:48:05.293273 7fa23b44aa80 MANIFEST file: MANIFEST-000937 size: 11591 Bytes
2019/08/23-13:48:05.293277 7fa23b44aa80 SST files in ./db/strings dir, Total Num: 179, files: 000089.sst 000573.sst 000577.sst 000581.sst 000585.sst 000606.sst 000667.sst 000668.sst 000680.sst
2019/08/23-13:48:05.293280 7fa23b44aa80 Write Ahead Log file in ./db/strings: 000932.log size: 35 ; 000935.log size: 0 ; 000938.log size: 0 ;
2019/08/23-13:48:05.293286 7fa23b44aa80 Options.error_if_exists: 0
2019/08/23-13:48:05.293288 7fa23b44aa80 Options.create_if_missing: 1
2019/08/23-13:48:05.293290 7fa23b44aa80 Options.paranoid_checks: 1
2019/08/23-13:48:05.293292 7fa23b44aa80 Options.env: 0xe534c0
2019/08/23-13:48:05.293294 7fa23b44aa80 Options.info_log: 0x28d4000
2019/08/23-13:48:05.293296 7fa23b44aa80 Options.max_file_opening_threads: 16
2019/08/23-13:48:05.293298 7fa23b44aa80 Options.statistics: (nil)
2019/08/23-13:48:05.293301 7fa23b44aa80 Options.use_fsync: 0
2019/08/23-13:48:05.293303 7fa23b44aa80 Options.max_log_file_size: 536870912
2019/08/23-13:48:05.293305 7fa23b44aa80 Options.max_manifest_file_size: 67108864
2019/08/23-13:48:05.293307 7fa23b44aa80 Options.log_file_time_to_roll: 0
2019/08/23-13:48:05.293308 7fa23b44aa80 Options.keep_log_file_num: 10
2019/08/23-13:48:05.293310 7fa23b44aa80 Options.recycle_log_file_num: 0
2019/08/23-13:48:05.293312 7fa23b44aa80 Options.allow_fallocate: 1
2019/08/23-13:48:05.293314 7fa23b44aa80 Options.allow_mmap_reads: 0
2019/08/23-13:48:05.293315 7fa23b44aa80 Options.allow_mmap_writes: 0
2019/08/23-13:48:05.293317 7fa23b44aa80 Options.use_direct_reads: 1
2019/08/23-13:48:05.293319 7fa23b44aa80 Options.use_direct_io_for_flush_and_compaction: 0
2019/08/23-13:48:05.293321 7fa23b44aa80 Options.create_missing_column_families: 0
2019/08/23-13:48:05.293322 7fa23b44aa80 Options.db_log_dir:
2019/08/23-13:48:05.293324 7fa23b44aa80 Options.wal_dir: ./db/strings
2019/08/23-13:48:05.293326 7fa23b44aa80 Options.table_cache_numshardbits: 6
2019/08/23-13:48:05.293328 7fa23b44aa80 Options.max_subcompactions: 3
2019/08/23-13:48:05.293330 7fa23b44aa80 Options.max_background_flushes: 2
2019/08/23-13:48:05.293331 7fa23b44aa80 Options.WAL_ttl_seconds: 0
2019/08/23-13:48:05.293333 7fa23b44aa80 Options.WAL_size_limit_MB: 0
2019/08/23-13:48:05.293335 7fa23b44aa80 Options.manifest_preallocation_size: 4194304
2019/08/23-13:48:05.293337 7fa23b44aa80 Options.is_fd_close_on_exec: 1
2019/08/23-13:48:05.293339 7fa23b44aa80 Options.advise_random_on_open: 1
2019/08/23-13:48:05.293340 7fa23b44aa80 Options.db_write_buffer_size: 0
2019/08/23-13:48:05.293342 7fa23b44aa80 Options.write_buffer_manager: 0x28c35f0
2019/08/23-13:48:05.293344 7fa23b44aa80 Options.access_hint_on_compaction_start: 1
2019/08/23-13:48:05.293346 7fa23b44aa80 Options.new_table_reader_for_compaction_inputs: 0
2019/08/23-13:48:05.293348 7fa23b44aa80 Options.compaction_readahead_size: 0
2019/08/23-13:48:05.293349 7fa23b44aa80 Options.random_access_max_buffer_size: 1048576
2019/08/23-13:48:05.293351 7fa23b44aa80
```
### creash reason
VersionSet::Recover -> ReadFileToString -> Read
If use direct reads, NewSequentialFile function will init fd, but FILE ptr is NULL. When invoke SequentialFile‘s Read, it will invoke fread_unlocked(scratch, 1, n, file_), but file_ is NULL, crash here!