efivars: make sure that _packed_ structure members are actually aligned as expected

efivars: make sure that _packed_ structure members are actually aligned as expected

When looking for the terminating double-NUL, don't just read the memory
until the terminator is found, but use the information we got about the
buffer size.

The length parameter passed to utf16_to_utf8() would include the terminator, so
the converted string would end up with two terminators (the original one
converted to "utf8", still 0, and then the one that was always added anyway).
Instead let's pass just the length of the actual data to utf16_to_utf8().