…16654)

Motivation:

Fix IllegalReferenceCountException in AdaptiveByteBuf.deallocate() when
JFR enabled when lot's of buffer is used, we noticed this error:

```
io.netty.util.IllegalReferenceCountException
    at AdaptiveByteBuf.rootParent(AdaptivePoolingAllocator.java:1653)
    at AdaptiveByteBuf.isDirect(AdaptivePoolingAllocator.java:1725)
    at AbstractBufferEvent.fill(AbstractBufferEvent.java:44)
    at AdaptiveByteBuf.deallocate(AdaptivePoolingAllocator.java:2110)
    at AbstractReferenceCountedByteBuf.handleRelease(AbstractReferenceCountedByteBuf.java:93)
    at AbstractReferenceCountedByteBuf.release(AbstractReferenceCountedByteBuf.java:83)
    at AdaptivePoolingAllocator$MagazineGroup.allocate(AdaptivePoolingAllocator.java:431)
    at AdaptivePoolingAllocator.allocate(AdaptivePoolingAllocator.java:271)
    at AdaptiveByteBufAllocator.newDirectBuffer(AdaptiveByteBufAllocator.java:67)

```
Modification:

When MagazineGroup.allocate() exhausts all magazine stripes under high
contention, the bail-out path calls buf.release() on an AdaptiveByteBuf
    that was obtained from newBuffer() but never initialised via init().
    The buffer's rootParent field is therefore null.

With JFR recording active, deallocate() fires a FreeBufferEvent before
    clearing rootParent. AbstractBufferEvent.fill() calls buf.isDirect()
    which delegates to rootParent().isDirect(), and rootParent() throws
    IllegalReferenceCountException when the field is null.

Result:

JFR events are emitted even under pressure

(cherry picked from commit 66568d1)