Fix SRP ciphersuite DoS vulnerability.

If a client attempted to use an SRP ciphersuite and it had not been
set up correctly it would crash with a null pointer read. A malicious
server could exploit this in a DoS attack.

Thanks to Joonas Kuorilehto and Riku Hietamäki from Codenomicon
for reporting this issue.

CVE-2014-2970
Reviewed-by: Tim Hudson <tjh@openssl.org>