varbin: Fix handle private field by accident

[xchacha20-poly1305]

• If a field named _, CanSet() returns false.
• Or (||) operation skips CanSet() check, causing panic.

[xchacha20-poly1305]

Another bug:

Write incorrectly applies reflect.Indirect to the input rawData before passing it to the internal write function. This causes a panic when a nil pointer is passed because reflect.Indirect returns an invalid reflect.Value, and the subsequent error handling path attempts to call .String() on a nil type.

Furthermore, stripping the pointer type at the top level causes the encoder to bypass the pointer-handling logic that writes a presence marker (0 or 1). Because the Read function expects this marker to determine if it should allocate memory, the lack of this byte causes a protocol mismatch, leading to data corruption or decoding failures.

Trying to fix this may breaks the protocol behavior, so I will not create a pull request to fix it.