Use BUFFER_SIZE instead of sizeof

The size of the username and passphrase buffers is BUFFER_SIZE, but
we're using sizeof to determine the size. This works since the buffers
are allocated as arrays but they could just as well be pointers. Use
BUFFER_SIZE instead of sizeof so that we always get the correct size of
the buffer.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>