Correctly detect b < a in BN_usub().

Correctly detect b < a in BN_usub().

BN_usub() requires that a >= b and should return an error in the case that
b < a. This is currently only detected by checking the number of words in
a versus b - if they have the same number of words, the top word is not
checked and b < a, which then succeeds and produces an incorrect result.

Fix this by checking for the case where a and b have an equal number of
words, yet there is a borrow returned from bn_sub_words().

ok miod@ tb@