The current implementation correctly copies up to 15 bytes of data, but the nullify code clears up to 16 bytes, which was likely motivated by the assumption that the length indicator is always the last byte.

This changes limits the nullify to 15 bytes as well removing this layout specific assumption from cord.h, making future platform specific internal layout changes easier to land.

PiperOrigin-RevId: 477997741
Change-Id: Idcdfeca2a005139f97eafcc77111542d90b817af