Fix assertion error with read_opts.iter_start_ts (#10279)

Summary:
If the internal iterator is not valid, `SeekToLast` with iter_start_ts should have `valid_` is false without assertion failure.
Test plan
make check

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10279

Reviewed By: ltamasi

Differential Revision: D37539393

Pulled By: riversand963

fbshipit-source-id: 8e94057838f8a05144fad5768f4d62f1893ec315

db/db_iter.cc

@@ -1612,7 +1612,7 @@ void DBIter::SeekToLast() {
     SeekForPrev(*iterate_upper_bound_);
     const bool is_ikey = (timestamp_size_ > 0 && timestamp_lb_ != nullptr);
     Slice k = Valid() ? key() : Slice();
-    if (is_ikey) {
+    if (is_ikey && Valid()) {
       k.remove_suffix(kNumInternalBytes + timestamp_size_);
     }
     while (Valid() && 0 == user_comparator_.CompareWithoutTimestamp(

db/db_with_timestamp_basic_test.cc

@@ -1193,6 +1193,14 @@ TEST_F(DBBasicTestWithTimestamp, SimpleBackwardIterateLowerTsBound) {
     it->SeekToLast();
     CheckIterEntry(it.get(), "a", kTypeSingleDeletion, Slice(),
                    Timestamp(1, 0));
+
+    key_ub_str = "a";  // exclusive
+    key_ub = key_ub_str;
+    read_opts.iterate_upper_bound = &key_ub;
+    it.reset(db_->NewIterator(read_opts));
+    it->SeekToLast();
+    ASSERT_FALSE(it->Valid());
+    ASSERT_OK(it->status());
   }
 
   Close();