Add a bit more validation in the ViewHistory constructor

- Ensure that `database.files` actually contains an Array, rather than some arbitrary data.

 - Only try to lookup an existing entry when the `database` existed on load, since there's obviously nothing to find when `database.files = []` was set (this case is very common in the MOZCENTRAL build since `sessionStorage` is being used there).

web/view_history.js

@@ -31,19 +31,20 @@ class ViewHistory {
 
     this._initializedPromise = this._readFromStorage().then(databaseStr => {
       const database = JSON.parse(databaseStr || "{}");
-      if (!("files" in database)) {
+      let index = -1;
+      if (!Array.isArray(database.files)) {
         database.files = [];
       } else {
         while (database.files.length >= this.cacheSize) {
           database.files.shift();
         }
-      }
-      let index = -1;
-      for (let i = 0, length = database.files.length; i < length; i++) {
-        const branch = database.files[i];
-        if (branch.fingerprint === this.fingerprint) {
-          index = i;
-          break;
+
+        for (let i = 0, ii = database.files.length; i < ii; i++) {
+          const branch = database.files[i];
+          if (branch.fingerprint === this.fingerprint) {
+            index = i;
+            break;
+          }
         }
       }
       if (index === -1) {