Router: fixed segmentation fault.

The router process exited abnormally on reconfiguration if number
of worker threads had been decreased on the previous reconfiguration.
Besides the list of router engines should be updated only after a new
configuration joints have been prepared for all engines.

src/nxt_router.h

@@ -34,6 +34,12 @@ typedef struct {
 typedef struct {
     nxt_event_engine_t     *engine;
     nxt_work_t             *jobs;
+
+    enum {
+        NXT_ROUTER_ENGINE_KEEP = 0,
+        NXT_ROUTER_ENGINE_ADD,
+        NXT_ROUTER_ENGINE_DELETE,
+    }                      action;
 } nxt_router_engine_conf_t;