BUG/MINOR: ssl: check conn in keylog sample fetch

Add a check on the conn pointer to avoid a NULL dereference in
smp_fetch_ssl_x_keylog().

The problem is not suppose to happen because the function is only used
for the frontend at the moment.

Introduced by 7d42ef5, 2.2 only.

Fix issue #733.
diff --git a/src/ssl_sample.c b/src/ssl_sample.c
index 843554c..e53c088 100644
--- a/src/ssl_sample.c
+++ b/src/ssl_sample.c
@@ -1121,6 +1121,9 @@
 	conn = (kw[4] != 'b') ? objt_conn(smp->sess->origin) :
 	       smp->strm ? cs_conn(objt_cs(smp->strm->si[1].end)) : NULL;
 
+	if (!conn)
+		return 0;
+
 	if (conn->flags & CO_FL_WAIT_XPRT) {
 		smp->flags |= SMP_F_MAY_CHANGE;
 		return 0;