fix corner case in unused (#3496)

fixes #3495

lib/compress.js

@@ -3629,6 +3629,7 @@ merge(Compressor.prototype, {
             return !(def.id in in_use_ids) || def.orig.length > 1;
         };
         // pass 3: we should drop declarations not in_use
+        var unused_fn_names = [];
         var tt = new TreeTransformer(function(node, descend, in_list) {
             var parent = tt.parent();
             if (drop_vars) {
@@ -3656,7 +3657,7 @@ merge(Compressor.prototype, {
             }
             if (scope !== self) return;
             if (node instanceof AST_Function && node.name && drop_fn_name(node.name.definition())) {
-                node.name = null;
+                unused_fn_names.push(node);
             }
             if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
                 var trim = compressor.drop_fargs(node, parent);
@@ -3853,6 +3854,9 @@ merge(Compressor.prototype, {
         });
         tt.push(compressor.parent());
         self.transform(tt);
+        unused_fn_names.forEach(function(fn) {
+            fn.name = null;
+        });
 
         function verify_safe_usage(def, read, modified) {
             if (def.id in in_use_ids) return;