The comments were about the time we were using mbedtls_pk_ec(), which
can return NULL, which we don't want to propagate to other functions.
Now we're using mbedtls_pk_get_ec_group_id() with is a safer interface
(and works even when EC is provided by drivers).
The check for GROUP_NONE was an heritage from the previous NULL check.
However it's actually useless: if NONE were returned (which can't happen
or parsing of the certificate would have failed and we wouldn't be
here), then mbedtls_ssl_check_curve() would work and just say that the
curve wasn't valid, which is OK.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>