Fix a crash when compaction fails to open a file

Summary:
We've got a crash with this stack trace:

  Program terminated with signal SIGTRAP, Trace/breakpoint trap.

  #0  0x00007fc85f2f4009 in raise () from /usr/local/fbcode/gcc-4.9-glibc-2.20-fb/lib/libpthread.so.0
  facebook#1  0x00000000005c8f61 in facebook::logdevice::handle_sigsegv(int) () at logdevice/server/sigsegv.cpp:159
  facebook#2  0x00007fc85f2f4150 in <signal handler called> () at /usr/local/fbcode/gcc-4.9-glibc-2.20-fb/lib/libpthread.so.0
  facebook#3  0x00000000031ed80c in rocksdb::NewReadaheadRandomAccessFile() at util/file_reader_writer.cc:383
  facebook#4  0x00000000031ed80c in rocksdb::NewReadaheadRandomAccessFile() at util/file_reader_writer.cc:472
  facebook#5  0x00000000031558e7 in rocksdb::TableCache::GetTableReader() at db/table_cache.cc:99
  facebook#6  0x0000000003156329 in rocksdb::TableCache::NewIterator() at db/table_cache.cc:198
  facebook#7  0x0000000003166568 in rocksdb::VersionSet::MakeInputIterator() at db/version_set.cc:3345
  facebook#8  0x000000000324a94f in rocksdb::CompactionJob::ProcessKeyValueCompaction(rocksdb::CompactionJob::SubcompactionState*) () at db/compaction_job.cc:650
  facebook#9  0x000000000324c2f6 in rocksdb::CompactionJob::Run() () at db/compaction_job.cc:530
  facebook#10 0x00000000030f5ae5 in rocksdb::DBImpl::BackgroundCompaction() at db/db_impl.cc:3269
  facebook#11 0x0000000003108d36 in rocksdb::DBImpl::BackgroundCallCompaction(void*) () at db/db_impl.cc:2970
  facebook#12 0x00000000029a2a9a in facebook::logdevice::RocksDBEnv::callback(void*) () at logdevice/server/locallogstore/RocksDBEnv.cpp:26
  facebook#13 0x00000000029a2a9a in facebook::logdevice::RocksDBEnv::callback(void*) () at logdevice/server/locallogstore/RocksDBEnv.cpp:30
  facebook#14 0x00000000031e7521 in rocksdb::ThreadPool::BGThread() at util/threadpool.cc:230
  facebook#15 0x00000000031e7663 in rocksdb::BGThreadWrapper(void*) () at util/threadpool.cc:254
  facebook#16 0x00007fc85f2ea7f1 in start_thread () at /usr/local/fbcode/gcc-4.9-glibc-2.20-fb/lib/libpthread.so.0
  facebook#17 0x00007fc85e8fb46d in clone () at /usr/local/fbcode/gcc-4.9-glibc-2.20-fb/lib/libc.so.6

From looking at the code, probably what happened is this:
 - `TableCache::GetTableReader()` called `Env::NewRandomAccessFile()`, which dispatched to a `PosixEnv::NewRandomAccessFile()`, where probably an `open()` call failed, so the `NewRandomAccessFile()` left a nullptr in the resulting file,
 - `TableCache::GetTableReader()` called `NewReadaheadRandomAccessFile()` with that `nullptr` file,
 - it tried to call file's method and crashed.

This diff is a trivial fix to this crash.

Test Plan: `make -j check`

Reviewers: sdong, andrewkr, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62451

db/table_cache.cc

@@ -95,11 +95,11 @@ Status TableCache::GetTableReader(
   unique_ptr<RandomAccessFile> file;
   Status s = ioptions_.env->NewRandomAccessFile(fname, &file, env_options);
 
-  if (readahead > 0) {
-    file = NewReadaheadRandomAccessFile(std::move(file), readahead);
-  }
   RecordTick(ioptions_.statistics, NO_FILE_OPENS);
   if (s.ok()) {
+    if (readahead > 0) {
+      file = NewReadaheadRandomAccessFile(std::move(file), readahead);
+    }
     if (!sequential_mode && ioptions_.advise_random_on_open) {
       file->Hint(RandomAccessFile::RANDOM);
     }