Fix unaligned read and write

See crbug.com/532120

BUG=chromium:532120

Run on chromeos checkout:
    FEATURES='test' USE='asan clang' \
        CXXFLAGS='-fsanitize=address -fsanitize=alignment' \
        emerge-x86-generic libweave

Change-Id: I817317e8670c46698fd7dbbd05d44798c541ce0b
Reviewed-on: https://weave-review.googlesource.com/1080
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
Reviewed-by: Gene Gutnik <gene@google.com>