Allow negative memoryAddress when calling PlatformDependent0.newDirectBuffer(...)

Motivation:

When UNSAFE.allocateMemory is returning an address whose high bit is set we currently throw an IllegalArgumentException. This is not correct as it may return a negative number on at least sparc.

Modifications:

- Allow to pass in negative memoryAddress
- Add unit tests

Result:

Correctly validate the memoryAddress and so also work on sparc as expected. Fixes [#6574].

common/src/main/java/io/netty/util/internal/PlatformDependent0.java

@@ -317,7 +317,6 @@ final class PlatformDependent0 {
     }
 
     static ByteBuffer newDirectBuffer(long address, int capacity) {
-        ObjectUtil.checkPositiveOrZero(address, "address");
         ObjectUtil.checkPositiveOrZero(capacity, "capacity");
 
         try {

common/src/test/java/io/netty/util/internal/PlatformDependent0Test.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2017 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.util.internal;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.nio.ByteBuffer;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assume.assumeTrue;
+
+public class PlatformDependent0Test {
+
+    @BeforeClass
+    public static void assumeUnsafe() {
+        assumeTrue(PlatformDependent0.hasUnsafe());
+    }
+
+    @Test
+    public void testNewDirectBufferNegativeMemoryAddress() {
+        testNewDirectBufferMemoryAddress(-1);
+    }
+
+    @Test
+    public void testNewDirectBufferNonNegativeMemoryAddress() {
+        testNewDirectBufferMemoryAddress(10);
+    }
+
+    @Test
+    public void testNewDirectBufferZeroMemoryAddress() {
+        PlatformDependent0.newDirectBuffer(0, 10);
+    }
+
+    private static void testNewDirectBufferMemoryAddress(long address) {
+        assumeTrue(PlatformDependent0.hasDirectBufferNoCleanerConstructor());
+
+        int capacity = 10;
+        ByteBuffer buffer = PlatformDependent0.newDirectBuffer(address, capacity);
+        assertEquals(address, PlatformDependent0.directBufferAddress(buffer));
+        assertEquals(capacity, buffer.capacity());
+    }
+}