BUG/MINOR: ssl: Fix OCSP_CERTID leak when same certificate is used multiple times

If a given certificate is used multiple times in a configuration, the
ocsp_cid field would have been overwritten during each
ssl_sock_load_ocsp call even if it was previously filled.

This patch does not need to be backported.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index efa31ea..18d006f 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -1150,7 +1150,8 @@
 	if (!issuer)
 		goto out;
 
-	data->ocsp_cid = OCSP_cert_to_id(0, x, issuer);
+	if (!data->ocsp_cid)
+		data->ocsp_cid = OCSP_cert_to_id(0, x, issuer);
 	if (!data->ocsp_cid)
 		goto out;