BUG/MINOR: ssl: clear the SSL errors on DH loading failure

BUG/MINOR: ssl: clear the SSL errors on DH loading failure

In ssl_sock_load_dh_params(), if haproxy failed to apply the dhparam
with SSL_CTX_set_tmp_dh(), it will apply the DH with
SSL_CTX_set_dh_auto().

The problem is that we don't clean the OpenSSL errors when leaving this
function so it could fail to load the certificate, even if it's only a
warning.

Fixes bug #483.

Must be backported in 2.1.

(cherry picked from commit 4dd145a888c7679812664bf2f246fa8199e94ab0)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>