EmptyByteBuf allows writing ByteBufs with 0 readable bytes

Motivation:

The contract of `ByteBuf.writeBytes(ByteBuf src)` is such that it will
throw an `IndexOutOfBoundsException if `src.readableBytes()` is greater than
`this.writableBytes()`. The EmptyByteBuf class will throw the exception,
even if the source buffer has zero readable bytes, in violation of the
contract.

Modifications:

Use the helper method `checkLength(..)` to check the length and throw
the exception, if appropriate.

Result:

Conformance with the stated behavior of ByteBuf.

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

@@ -760,7 +760,7 @@ public final class EmptyByteBuf extends ByteBuf {
 
     @Override
     public ByteBuf writeBytes(ByteBuf src) {
-        throw new IndexOutOfBoundsException();
+        return checkLength(src.readableBytes());
     }
 
     @Override

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

@@ -28,6 +28,19 @@ public class EmptyByteBufTest {
         assertFalse(empty.isWritable(1));
     }
 
+    @Test
+    public void testWriteEmptyByteBuf() {
+        EmptyByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);
+        empty.writeBytes(Unpooled.EMPTY_BUFFER); // Ok
+        ByteBuf nonEmpty = UnpooledByteBufAllocator.DEFAULT.buffer().writeBoolean(false);
+        try {
+            empty.writeBytes(nonEmpty);
+            fail();
+        } catch (IndexOutOfBoundsException ignored) {
+            // Ignore.
+        }
+    }
+
     @Test
     public void testIsReadable() {
         EmptyByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);