Make sure that gr-router#finalize is safe to be called more than once.

This is necessary as GrRouter is registered under two different
tokens.

Release-Notes: skip
Change-Id: Iaa3a0b7e81c97abdc33fd373540771511bb3d97b
diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
index 2bda156..b90ef91 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
@@ -343,6 +343,7 @@
     for (const subscription of this.subscriptions) {
       subscription.unsubscribe();
     }
+    this.subscriptions = [];
   }
 
   start() {