Fix EC_POINT_bn2point() for BN_zero()

Fix EC_POINT_bn2point() for BN_zero()

EC_POINT_bn2point() rejected BIGNUMs with a zero value.

This behavior indirectly caused failures when converting a point
at infinity through EC_POINT_point2hex() and then back to a point with
EC_POINT_hex2point().

With this change such BIGNUMs are treated like any other and exported to
an octet buffer filled with zero.
It is then EC_POINT_oct2point() (either the default implementation or
the custom one in group->meth->oct2point) to determine if such encoding
maps to a valid point (generally the point at infinity is encoded as
0x00).

Fixes #10258

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10329)