CLEANUP: backend: shut another false null-deref in back_handle_st_con()

objt_conn() may return a NULL though here we don't have this situation
anymore since the connection is always there, so let's simply switch
to the unchecked __objt_conn(). This addresses issue #454.
diff --git a/src/backend.c b/src/backend.c
index 859a004..ed2c969 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1916,7 +1916,7 @@
 	struct channel *req = &s->req;
 	struct channel *rep = &s->res;
 	struct conn_stream *srv_cs = objt_cs(si->end);
-	struct connection *conn = srv_cs ? srv_cs->conn : objt_conn(si->end);
+	struct connection *conn = srv_cs ? srv_cs->conn : __objt_conn(si->end);
 
 	DBG_TRACE_ENTER(STRM_EV_STRM_PROC|STRM_EV_SI_ST, s);