Fixed a crash in debug build in flush_job.cc

Summary:
It was doing `&range_del_iters[0]` on an empty vector. Even though the resulting pointer is never dereferenced, it's still bad for two reasons:
* the practical reason: it crashes with `std::out_of_range` exception in our debug build,
* the "C++ standard lawyer" reason: it's undefined behavior because, in `std::vector` implementation, it probably "dereferences" a null pointer, which is invalid even though it doesn't actually read the pointed memory, just converts a pointer into a reference (and then flush_job.cc converts it back to pointer); nullptr references are undefined behavior.
Closes https://github.com/facebook/rocksdb/pull/1612

Differential Revision: D4265625

Pulled By: al13n321

fbshipit-source-id: db26fb9

db/flush_job.cc

@@ -277,7 +277,8 @@ Status FlushJob::WriteLevel0Table() {
           NewMergingIterator(&cfd_->internal_comparator(), &memtables[0],
                              static_cast<int>(memtables.size()), &arena));
       std::unique_ptr<InternalIterator> range_del_iter(NewMergingIterator(
-          &cfd_->internal_comparator(), &range_del_iters[0],
+          &cfd_->internal_comparator(),
+          range_del_iters.empty() ? nullptr : &range_del_iters[0],
           static_cast<int>(range_del_iters.size())));
       Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
           "[%s] [JOB %d] Level-0 flush table #%" PRIu64 ": started",