Motivation:
We synchronize on the chunk.arena when produce the String returned by PoolSubpage.toString() which may raise a NPE when chunk == null. Chunk == null for the head of the linked-list and so a NPE may raised by a debugger. This NPE can never happen in real code tho as we never access toString() of the head.
Modifications:
Add null checks and so fix the possible NPE
Result:
No NPE when using a debugger and inspect the PooledByteBufAllocator.
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