Don't fail if a key we want to delete is already deleted

This can happen when cleaning up stale users at boot time.

Bug: 30158800
Change-Id: I2733d8d525fc79b7f05eb2225b7e6e14c4da277f

Ext4Crypt.cpp

@@ -534,7 +534,12 @@ bool e4crypt_destroy_user_key(userid_t user_id) {
         for (auto const path: get_ce_key_paths(get_ce_key_directory_path(user_id))) {
             success &= android::vold::destroyKey(path);
         }
-        success &= android::vold::destroyKey(get_de_key_path(user_id));
+        auto de_key_path = get_de_key_path(user_id);
+        if (path_exists(de_key_path)) {
+            success &= android::vold::destroyKey(de_key_path);
+        } else {
+            LOG(INFO) << "Not present so not erasing: " << de_key_path;
+        }
     }
     return success;
 }