harden just a bit more the migration code

platform/webext/from-legacy.js

@@ -27,21 +27,26 @@
 
 (function() {
     let µb = µBlock;
+    let migratedKeys = new Set();
 
     let migrateAll = function(callback) {
-        let mustRestart = false;
-
         let migrateKeyValue = function(details, callback) {
+            // https://github.com/gorhill/uBlock/issues/2653
+            // Be ready to deal graciously with corrupted DB.
+            if ( migratedKeys.has(details.key) ) {
+                callback();
+                return;
+            }
+            migratedKeys.add(details.key);
             let bin = {};
             bin[details.key] = JSON.parse(details.value);
             self.browser.storage.local.set(bin, callback);
-            mustRestart = true;
         };
 
         let migrateNext = function() {
             self.browser.runtime.sendMessage({ what: 'webext:storageMigrateNext' }, response => {
                 if ( response.key === undefined ) {
-                    if ( mustRestart ) {
+                    if ( migratedKeys.size !== 0 ) {
                         self.browser.runtime.reload();
                     } else {
                         callback();