make 'cache = null' to help PoolThreadCache finalizer. (#12881)

Motivation:
PoolByteBuf deallocate doesn't set `cache = null` when deallocate PoolByteBuf, so the PollThreadCache is always reachable by GC root. It is reachable until the cache is overridden at the next init.

Modifications:
Set cache to null when `deallocate()` method is called.

Result:
More timely deallocation of `PoolThreadCache` possible.

Author: horizonzy

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

@@ -176,6 +176,7 @@ protected final void deallocate() {
             chunk.arena.free(chunk, tmpNioBuf, handle, maxLength, cache);
             tmpNioBuf = null;
             chunk = null;
+            cache = null;
             recycle();
         }
     }