dsa/dsa_gen: add error message for seed_len < 0

prio openssl 1.1.0 seed_len < q was accepted and the seed argument was
then ignored. Now DSA_generate_parameters_ex() returns an error in such
a case but no error string.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1657)
(cherry picked from commit af547412)