ECPublicKey_set_octet_string and ECPublicKey_get_octet_string

behaviour was not quite consistent with the conventions
for d2i and i2d functions as far as handling of the 'out'
or 'in' pointer is concerned.

This patch changes this behaviour, and renames the functions to
o2i_ECPublicKey and i2o_ECPublicKey (not 'd2i' and 'i2d' because the
external encoding is just a raw object string without any DER icing).

Submitted by: Nils Larsch