Check for EVP_MD being NULL inside ssl.

Check for EVP_MD being NULL inside ssl.

Fix multiple places that could potentially segfault if memory
allocations fail. e.g. ssl_load_ciphers() could fail while calling
ssl_evp_md_fetch().

Found by #18355

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/18784)