gui: Fix undefined lastSeenDays error in disconnected-inactive status…

… check (ref #8530) (#8730)

The current code assumes that lastSeenDays is always set which is not
the case when device reports its lastSeen as equal to the Unix epoch.
Thus, make sure that lastSeenDays is set before proceeding with the
disconnected-inactive status check to avoid JS errors in the browser.

Signed-off-by: Tomasz Wilczyński <twilczynski@naver.com>

gui/default/syncthing/core/syncthingController.js

@@ -1098,7 +1098,7 @@ angular.module('syncthing.core')
             }
 
             // Disconnected
-            if (!unused && $scope.deviceStats[deviceCfg.deviceID].lastSeenDays >= 7) {
+            if (!unused && $scope.deviceStats[deviceCfg.deviceID].lastSeenDays && $scope.deviceStats[deviceCfg.deviceID].lastSeenDays >= 7) {
                 return status + 'disconnected-inactive';
             } else {
                 return status + 'disconnected';