Prevent segfault in OpenCompactionOutputFile

Summary:
The segfault was happening because the program was unable to open a new
sst file (as part of the compaction) because the process ran out of
file descriptors.

The fix is to check the return status of the file creation before taking
any other action.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fabf03f9700 (LWP 29904)]
leveldb::DBImpl::OpenCompactionOutputFile (this=this@entry=0x7fabf9011400, compact=compact@entry=0x7fabf741a2b0) at db/db_impl.cc:1399
1399    db/db_impl.cc: No such file or directory.
(gdb) where

Test Plan: make check

Reviewers: MarkCallaghan, sheki

Reviewed By: MarkCallaghan

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10101

db/db_impl.cc

@@ -1393,12 +1393,12 @@ Status DBImpl::OpenCompactionOutputFile(CompactionState* compact) {
   std::string fname = TableFileName(dbname_, file_number);
   Status s = env_->NewWritableFile(fname, &compact->outfile, storage_options_);
 
-  // Over-estimate slightly so we don't end up just barely crossing
-  // the threshold.
-  compact->outfile->SetPreallocationBlockSize(
-    1.1 * versions_->MaxFileSizeForLevel(compact->compaction->level() + 1));
-
   if (s.ok()) {
+    // Over-estimate slightly so we don't end up just barely crossing
+    // the threshold.
+    compact->outfile->SetPreallocationBlockSize(
+      1.1 * versions_->MaxFileSizeForLevel(compact->compaction->level() + 1));
+
     compact->builder.reset(new TableBuilder(options_, compact->outfile.get(),
                                             compact->compaction->level() + 1));
   }