Fix explicit EC curve encoding.

Per SEC 1, the curve coefficients must be padded up to size. See C.2's
definition of Curve, C.1's definition of FieldElement, and 2.3.5's definition
of how to encode the field elements in http://www.secg.org/sec1-v2.pdf

.

This comes up for P-521, where b needs a leading zero.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6314)