Fix ZstdOutputStream corruption on double close

ZstdOutputStream will write out the last chunk every time close() is
invoked on it, which can cause errors when the output is later
decompressed. Per the java.io.Closeable interface documentation, the close() method
should have no effect if invoked on an already-closed stream (which is
how e.g. the core DeflaterOutputStream behaves as well), so make it a
noop if the stream was already closed.

src/main/java/io/airlift/compress/zstd/ZstdOutputStream.java

@@ -133,18 +133,22 @@ private void compressIfNecessary()
     void finishWithoutClosingSource()
             throws IOException
     {
-        writeChunk(true);
-        closed = true;
+        if (!closed) {
+            writeChunk(true);
+            closed = true;
+        }
     }
 
     @Override
     public void close()
             throws IOException
     {
-        writeChunk(true);
+        if (!closed) {
+            writeChunk(true);
 
-        closed = true;
-        outputStream.close();
+            closed = true;
+            outputStream.close();
+        }
     }
 
     private void writeChunk(boolean lastChunk)