BUG/MINOR: backend: Fix a possible null pointer dereference

In the function connect_server(), when we are not able to reuse a connection and
too many FDs are opened, the variable srv must be defined to kill an idle
connection.

This patch fixes the issue #257. It must be backported to 2.0
diff --git a/src/backend.c b/src/backend.c
index 1b01536..f03b1ab 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1268,7 +1268,7 @@
 	}
 
 	if (((!reuse || (srv_conn && !(srv_conn->flags & CO_FL_CONNECTED)))
-	    && ha_used_fds > global.tune.pool_high_count) && srv->idle_orphan_conns) {
+	    && ha_used_fds > global.tune.pool_high_count) && srv && srv->idle_orphan_conns) {
 		struct connection *tokill_conn;
 
 		/* We can't reuse a connection, and e have more FDs than deemd