BUILD/MINOR: ssl: fix build failure introduced by recent patch

Baptiste reported that commit 0a9a2b8 ("MEDIUM: sample change the
prototype of sample-fetches and converters functions") broke the
build of ssl_sock.c when using openssl-1.0.2 because one missed
replacement of sess with smp->sess. No backport is needed.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index d51d782..6cfdc73 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -3740,7 +3740,7 @@
 	smp->flags = SMP_F_CONST;
 	smp->type = SMP_T_STR;
 
-	conn = objt_conn(sess->origin);
+	conn = objt_conn(smp->sess->origin);
 	if (!conn || !conn->xprt_ctx || conn->xprt != &ssl_sock)
 		return 0;