MINOR: actually throw in PooledByteBufAllocatorL

There was a check in that class that looks like it throws
an exception, but actually does not. This means that when
that branch is hit, instead of an actionable error, the
user gets a ClassCastException like

```
java.lang.ClassCastException: class io.netty.buffer.PooledDirectByteBuf
cannot be cast to class io.netty.buffer.PooledUnsafeDirectByteBuf
(io.netty.buffer.PooledDirectByteBuf and io.netty.buffer.PooledUnsafeDirectByteBuf
are in unnamed module of loader org.springframework.boot.loader.launch.LaunchedClassLoader@5e9f23b4)
```

Actually throwing the exception prevents this.

Author: no23reason

memory/memory-netty-buffer-patch/src/main/java/io/netty/buffer/PooledByteBufAllocatorL.java

@@ -172,7 +172,7 @@ private UnsafeDirectLittleEndian newDirectBufferL(int initialCapacity, int maxCa
           // within chunk, use arena.
           ByteBuf buf = directArena.allocate(cache, initialCapacity, maxCapacity);
           if (!(buf instanceof PooledUnsafeDirectByteBuf)) {
-            fail();
+            throw fail();
           }
 
           if (!AssertionUtil.ASSERT_ENABLED) {