_helpers.c: be careful about memory alignment in bloom operations.

Debian (Bug#641488) discovered via the bloom test that bup crashes on
sparc because it's trying to access unaligned memory.  Fix that.

Signed-off-by: Julien Cristau <jcristau@debian.org>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
DESIGN: fix typo.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>