Add more tests to EmptyByteBufTest

- Ensure an EmptyByteBuf has an array, an NIO buffer, and a memory
  address at the same time
- Add an assertion that checks if EMPTY_BUFFER is an EmptyByteBuf,
  just in case we make a mistake in the future

buffer/src/main/java/io/netty/buffer/Unpooled.java

@@ -93,6 +93,10 @@ public final class Unpooled {
      */
     public static final ByteBuf EMPTY_BUFFER = ALLOC.buffer(0, 0);
 
+    static {
+        assert EMPTY_BUFFER instanceof EmptyByteBuf: "EMPTY_BUFFER must be an EmptyByteBuf.";
+    }
+
     /**
      * Creates a new big-endian Java heap buffer with reasonably small initial capacity, which
      * expands its capacity boundlessly on demand.

buffer/src/test/java/io/netty/buffer/EmptyByteBufTest.java

@@ -17,6 +17,9 @@
 import org.junit.Assert;
 import org.junit.Test;
 
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
 public class EmptyByteBufTest {
 
     @Test
@@ -32,4 +35,37 @@ public class EmptyByteBufTest {
         Assert.assertFalse(empty.isReadable());
         Assert.assertFalse(empty.isReadable(1));
     }
+
+    @Test
+    public void testArray() {
+        EmptyByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);
+        assertThat(empty.hasArray(), is(true));
+        assertThat(empty.array().length, is(0));
+        assertThat(empty.arrayOffset(), is(0));
+    }
+
+    @Test
+    public void testNioBuffer() {
+        EmptyByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);
+        assertThat(empty.nioBufferCount(), is(1));
+        assertThat(empty.nioBuffer().position(), is(0));
+        assertThat(empty.nioBuffer().limit(), is(0));
+        assertThat(empty.nioBuffer(), is(sameInstance(empty.nioBuffer())));
+        assertThat(empty.nioBuffer(), is(sameInstance(empty.internalNioBuffer(0, 0))));
+    }
+
+    @Test
+    public void testMemoryAddress() {
+        EmptyByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);
+        if (empty.hasMemoryAddress()) {
+            assertThat(empty.memoryAddress(), is(not(0L)));
+        } else {
+            try {
+                empty.memoryAddress();
+                fail();
+            } catch (UnsupportedOperationException ignored) {
+                // Ignore.
+            }
+        }
+    }
 }