message/unpack: Fix undefined behavior

It's UB to shift an integer by the size of that integer. More
specifically, if you disable compiler optimization and try and unpack a
32 bit bitset you will end up with a 0x0 mask. Avoid UB by replacing
shift subtract with a negate shift.

Tested:
    Unit tests pass now.

Change-Id: I03a6f866a51c955b57787d641da9180841747e4c
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed