Correct sign handling in BN_add_word().

Correct sign handling in BN_add_word().

A sign handling bug was introduced to BN_add_word() in bn_word.c r1.18.
When handling addition to a negative bignum, the BN_sub_word() call can
result in the sign being flipped, which we need to account for. Use the
same code in BN_sub_word() - while not technically needed here it keeps
the code consistent.

Issue discovered by tb@

ok tb@