rgw: Initialize pointer field in RGWObjectExpirer

Fixes the Coverity Scan Report:
CID 1351737 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
2. uninit_member: Non-static class member worker is not initialized in this constructor nor in any functions that it calls.

Signed-off-by: Jos Collin <jcollin@redhat.com>

src/rgw/rgw_object_expirer_core.h

@@ -65,7 +65,7 @@ class RGWObjectExpirer {
     void stop();
   };
 
-  OEWorker *worker;
+  OEWorker *worker{nullptr};
   std::atomic<bool> down_flag = { false };
 
 public: