OpenSSL: Handle EC_POINT_is_on_curve() error case
Even though this OpenSSL function is documented as returning "1 if point
if on the curve and 0 otherwise", it can apparently return -1 on some
error cases. Be prepared for that and check explicitly against 1 instead
of non-zero.
Signed-off-by: Jouni Malinen <j@w1.fi>