Use alloc().heapBuffer(...) to allocate new heap buffer.

Motivation

Underlying array allocations in UnpooledHeapByteBuf are intended be done
via the protected allocateArray(int) method, so that they can be tracked
and/or overridden by subclasses, for example
UnpooledByteBufAllocator$InstrumentedUnpooledHeapByteBuf or #8015. But
it looks like an explicit allocation was missed in the copy(int,int)
method.

Modification

Just use alloc().heapBuffer(...) for the allocation

Result

No possibility of "missing" array allocations when ByteBuf#copy is used.

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

@@ -536,9 +536,7 @@ public class UnpooledHeapByteBuf extends AbstractReferenceCountedByteBuf {
     @Override
     public ByteBuf copy(int index, int length) {
         checkIndex(index, length);
-        byte[] copiedArray = PlatformDependent.allocateUninitializedArray(length);
-        System.arraycopy(array, index, copiedArray, 0, length);
-        return new UnpooledHeapByteBuf(alloc(), copiedArray, maxCapacity());
+        return alloc().heapBuffer(length, maxCapacity()).writeBytes(array, index, length);
     }
 
     private ByteBuffer internalNioBuffer() {