Rewrite BN_{asc,dec,hex}2bn() using CBS.

Rewrite BN_{asc,dec,hex}2bn() using CBS.

This gives us more readable and safer code. There are two intentional
changes to behaviour - firstly, all three functions zero any BN that was
passed in, prior to doing any further processing. This means that a passed
BN is always in a known state, regardless of what happens later. Secondly,
BN_asc2bn() now fails on NULL input, rather than crashing. This brings its
behaviour inline with BN_dec2bn() and BN_hex2bn().

ok tb@