Fix presumed network filter not being a valid network filter

Related feedback:
- https://github.com/uBlockOrigin/uBlock-issues/issues/2560#issuecomment-1492959500

src/js/static-filtering-parser.js

@@ -1126,6 +1126,7 @@ export class AstFilterParser {
         if ( tail !== 0 ) {
             this.linkRight(prev, tail);
         }
+        if ( this.astType !== AST_TYPE_NETWORK ) { return 0; }
         this.validateNet();
         return this.throwHeadNode(head);
     }