This patch fixes a bug in the padding of umb strings. Instead of
padding the right position, umb_padding() would always zero padding
bytes at the beginning of the buffer.
For the two callers of umb_addstr(), this won't hurt in
umb_send_connect() since the first value in the buffer is the
session id, which is zero anyway. But for umb_setpin() we might try to
change the wrong type of PIN, iff the length of the PIN is not a
multiple of 4.