Don't throw if null is given as ByteBuf when adding components. (#11613)

Motivation:

232c669fa413a9079ee6216056b57a647143f4b6 did add some overflow protection but did not handle null elements in the array the same as before.

Modifications:

- Break the loop if a null element was found
- Add unit test

Result:

Fixes https://github.com/netty/netty/issues/11612

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

@@ -375,7 +375,11 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf implements
         int readableBytes = 0;
         int capacity = capacity();
         for (int i = arrOffset; i < buffers.length; i++) {
-            readableBytes += buffers[i].readableBytes();
+            ByteBuf b = buffers[i];
+            if (b == null) {
+                break;
+            }
+            readableBytes += b.readableBytes();
 
             // Check if we would overflow.
             // See https://github.com/netty/netty/issues/10194

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

@@ -1576,6 +1576,20 @@ public abstract class AbstractCompositeByteBufTest extends AbstractByteBufTest {
         assertTrue(cbuf.release());
     }
 
+    // See https://github.com/netty/netty/issues/11612
+    @Test
+    public void testAddComponentWithNullEntry() {
+        final ByteBuf buffer = Unpooled.buffer(8).writeZero(8);
+        final CompositeByteBuf compositeByteBuf = compositeBuffer(Integer.MAX_VALUE);
+        try {
+            compositeByteBuf.addComponents(true, new ByteBuf[] { buffer, null });
+            assertEquals(8, compositeByteBuf.readableBytes());
+            assertEquals(1, compositeByteBuf.numComponents());
+        } finally {
+            compositeByteBuf.release();
+        }
+    }
+
     @Test
     public void testOverflowWhileAddingComponent() {
         int capacity = 1024 * 1024; // 1MB