TIMINGSAFE*: slight bugfix for timingsafe_bcmp, and small change for timingsafe_memcmp.

timingsafe_bcmp => turning the predecrement to postdecrement gives expected behavior, plus a slight "optimization" for timingsafe_memcmp, withdraws an unnecessary if as if n equals 0 the following computation does not occur anyway ...

Signed-off-by: Shawn Wenn <lattera@gmail.com>