Makes `EmptyByteBuf#hashCode` and `AbstractByteBuf#hashCode` consistent (#7870)

Motivation:
The `AbstractByteBuf#equals` method doesn't take into account the
class of buffer instance. So the two buffers with different classes
must have the same `hashCode` values if `equals` method returns `true`.
But `EmptyByteBuf#hashCode` is not consistent with `#hashCode`
of the empty `AbstractByteBuf`, that is violates the contract and
can lead to errors.

Modifications:
Return `1` in `EmptyByteBuf#hashCode`.

Result:
Consistent behavior of `EmptyByteBuf#hashCode` and `AbstractByteBuf#hashCode`.

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

@@ -168,7 +168,7 @@ public final class ByteBufUtil {
         final int intCount = aLen >>> 2;
         final int byteCount = aLen & 3;
 
-        int hashCode = 1;
+        int hashCode = EmptyByteBuf.EMPTY_BYTE_BUF_HASH_CODE;
         int arrayIndex = buffer.readerIndex();
         if (buffer.order() == ByteOrder.BIG_ENDIAN) {
             for (int i = intCount; i > 0; i --) {

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

@@ -36,6 +36,7 @@ import java.nio.charset.Charset;
  */
 public final class EmptyByteBuf extends ByteBuf {
 
+    static final int EMPTY_BYTE_BUF_HASH_CODE = 1;
     private static final ByteBuffer EMPTY_BYTE_BUFFER = ByteBuffer.allocateDirect(0);
     private static final long EMPTY_BYTE_BUFFER_ADDRESS;
 
@@ -976,7 +977,7 @@ public final class EmptyByteBuf extends ByteBuf {
 
     @Override
     public int hashCode() {
-        return 0;
+        return EMPTY_BYTE_BUF_HASH_CODE;
     }
 
     @Override

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

@@ -82,4 +82,15 @@ public class EmptyByteBufTest {
             }
         }
     }
+
+    @Test
+    public void consistentEqualsAndHashCodeWithAbstractBytebuf() {
+        ByteBuf empty = new EmptyByteBuf(UnpooledByteBufAllocator.DEFAULT);
+        ByteBuf emptyAbstract = new UnpooledHeapByteBuf(UnpooledByteBufAllocator.DEFAULT, 0, 0);
+        assertEquals(emptyAbstract, empty);
+        assertEquals(emptyAbstract.hashCode(), empty.hashCode());
+        assertEquals(EmptyByteBuf.EMPTY_BYTE_BUF_HASH_CODE, empty.hashCode());
+        assertTrue(emptyAbstract.release());
+        assertFalse(empty.release());
+    }
 }