Minor fix in rocksdb jni library.

Summary: Fix a bug in RocksDBSample.java and minor code improvement in rocksjni.cc.

Test Plan:
make jni
make jtest

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17325

java/org/rocksdb/RocksDB.java

@@ -34,7 +34,9 @@ public class RocksDB implements Closeable {
   }
 
   @Override public void close() throws IOException {
-    close0();
+    if (nativeHandle != 0) {
+      close0();
+    }
   }
 
   /**