Exception thrown from getApproximateMemTableStats when less than 2 records in the range

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In rocksdb java[6.13.3], when calling getApproximateMemTableStats with a range containing 0 or 1 record, following exception will be thrown:

java.lang.ArrayIndexOutOfBoundsException: 1
at org.rocksdb.RocksDB.getApproximateMemTableStats(RocksDB.java:3147)
at org.rocksdb.RocksDB.getApproximateMemTableStats(RocksDB.java:3161)

### Expected behavior
No exception thrown
### Actual behavior
ArrayIndexOutOfBoundsException thrown

### Steps to reproduce the behavior
```
@Test
public void getApproximateMemTableStats() throws RocksDBException {
final byte key1[] = "key1".getBytes(UTF_8);
final byte key2[] = "key2".getBytes(UTF_8);
final byte key3[] = "key3".getBytes(UTF_8);
try (final Options options = new Options().setCreateIfMissing(true)) {
final String dbPath = dbFolder.getRoot().getAbsolutePath();

try (final RocksDB db = RocksDB.open(options, dbPath)) {
db.put(key1, key1);
// db.put(key2, key2); <-- uncomment this line will make the test pass

final RocksDB.CountAndSize stats = db.getApproximateMemTableStats(new Range(new Slice(key1), new Slice(key3)));
assertNotNull(stats);
assertTrue(stats.count > 1);
assertTrue(stats.size > 1);
}
}
}
```