Set up callback only after target_list is loaded (#18)

This fixes an issue observed in Kong: when restarting the kong process, the concurrent worker processes load targets, and trigger other workers's event callback. When callbacks hit before the target list is ready, the event_handler method error would happen at line 925: `the targets is nil` when indexing its `ip` field.

See Kong/kong#4453

lib/resty/healthcheck.lua

@@ -1267,12 +1267,6 @@ function _M.new(opts)
   -- register for events, and directly after load initial target list
   -- order is important!
   do
-    self.ev_callback = function(data, event)
-      -- just a wrapper to be able to access `self` as a closure
-      return self:event_handler(event, data.ip, data.port, data.hostname)
-    end
-    worker_events.register_weak(self.ev_callback, self.EVENT_SOURCE)
-
     -- Lock the list, in case it is being cleared by another worker
     local ok, err = locking_target_list(self, function(target_list)
 
@@ -1296,6 +1290,12 @@ function _M.new(opts)
       self:log(ERR, "Error loading initial target list: ", err)
     end
 
+    self.ev_callback = function(data, event)
+      -- just a wrapper to be able to access `self` as a closure
+      return self:event_handler(event, data.ip, data.port, data.hostname)
+    end
+    worker_events.register_weak(self.ev_callback, self.EVENT_SOURCE)
+
     -- handle events to sync up in case there was a change by another worker
     worker_events:poll()
   end