efi: as extra paranoia NUL terminate UTF-16 strings with three NUL bytes

efi: as extra paranoia NUL terminate UTF-16 strings with three NUL bytes

This is a safey net anyway, let's make it fully safe: if the data ends
on an uneven byte, then we need to complete the UTF-16 codepoint first,
before adding the final NUL byte pair. Hence let's suffix with three
NULs, instead of just two.

(cherry picked from commit c75e7da0b53dd67363b724502e91901e89097886)