osd: initialize class_handler of OSD properly

The destructor of OSD will delete class_handler arbitrarily
and there are cases we'll fail to start an osd before we
load the class_handler correctly.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/osd/OSD.h

@@ -1234,7 +1234,7 @@ class OSD : public Dispatcher,
   bool asok_command(string command, cmdmap_t& cmdmap, string format, ostream& ss);
 
 public:
-  ClassHandler  *class_handler;
+  ClassHandler  *class_handler = nullptr;
   int get_nodeid() { return whoami; }
 
   static ghobject_t get_osdmap_pobject_name(epoch_t epoch) {