Ensure all members are initialized before loading selfie

Theoretically, if a selfie is loaded from corrupted
storage resulting in a thrown exception from the
constructor, this would cause an improperly initialized
HostnameBasedDB instance and overall potentially a
broken uBO.

Related feedback:
- https://bugzilla.mozilla.org/show_bug.cgi?id=1543880#c7

src/js/static-ext-filtering.js

@@ -516,14 +516,14 @@
             this.nBits = nBits;
             this.timer = undefined;
             this.strToIdMap = new Map();
-            if ( selfie !== undefined ) {
-                this.fromSelfie(selfie);
-                return;
-            }
             this.hostnameToSlotIdMap = new Map();
             this.hostnameSlots = [];
             this.strSlots = [];
             this.size = 0;
+            if ( selfie !== undefined ) {
+                this.fromSelfie(selfie);
+                return;
+            }
         }
 
         store(hn, bits, s) {