Use BN_bn2binpad() instead of handrolling it

Use BN_bn2binpad() instead of handrolling it

As ugly as the BN_bn2binpad() internals are, what it does is quite handy
with all sorts of EC stuff. So use it here too and eliminate some ugly
manual pointer zeroing and offsets. Also switch len and buflen from size_t
to int to remove an iffy cast: both are set by functions that return a
non-negative int.

ok jsing