Fix a crash when service shutdown is signaled and object API is not ready yet (#2939)

cmd/service.go

@@ -113,9 +113,10 @@ func (m *ServerMux) handleServiceSignals() error {
 				if objAPI == nil {
 					// Server not initialized yet, exit happily.
 					runExitFn(nil)
-				}
+				} else {
 					runExitFn(objAPI.Shutdown())
 				}
 			}
 		}
+	}
 }