fix(targetlist) avoid self.targets being nil due to async behaviour (#44)

when two worker create `checker`, `worker-0` first created and add
target success, then `worker-1` creating, when `worker-1` call
`worker_events:poll()` in line 1408, raise crash.

`self.targets` init with empty table avoid this.

Author: juzipeek

lib/resty/healthcheck.lua

@@ -1760,7 +1760,7 @@ function _M.new(opts)
   end
 
   -- other properties
-  self.targets = nil     -- list of targets, initially loaded, maintained by events
+  self.targets = {}     -- list of targets, initially loaded, maintained by events
   self.events = nil      -- hash table with supported events (prevent magic strings)
   self.ev_callback = nil -- callback closure per checker instance