[SAY-5]

getxattr and listxattr return the attribute size when the buffer is too small, so with an empty &mut [MaybeUninit<u8>] the length handed to assume_init exceeds the buffer and split_at_mut panics with mid > len. This clamps the initialized length to the buffer length in the MaybeUninit impls, the same way recv already handles MSG_TRUNC; &mut [u8] buffers still report the full size.

Fixes #1693