Fix regression in selfie destruction code

Related commit:
- 915687fddb (diff-73ef8c4664f2ec8c02320d50b2908efdR1100-R1113)

Since selfie destruction is now deferred so as to
coallesce burst of call to destroy(), the selfie
load code must mind whether there is a pending
destruction in order to decide whether the
selfie can be safely loaded.

Related feedback:
- 23c4c80136 (commitcomment-35179834)

src/js/background.js

@@ -140,8 +140,8 @@ const µBlock = (( ) => { // jshint ignore:line
 
         // Read-only
         systemSettings: {
-            compiledMagic: 19,  // Increase when compiled format changes
-            selfieMagic: 19,    // Increase when selfie format changes
+            compiledMagic: 20,  // Increase when compiled format changes
+            selfieMagic: 20,    // Increase when selfie format changes
         },
 
         restoreBackupSettings: {

src/js/storage.js

@@ -1052,6 +1052,10 @@
     };
 
     const load = async function() {
+        if ( destroyTimer !== undefined ) {
+            return Promise.resolve(false);
+        }
+
         return Promise.all([
             µb.assets.get('selfie/main').then(details => {
                 if (