fix corner case in collapse_vars (#4855)

fixes #4854

lib/compress.js

@@ -1792,7 +1792,12 @@ merge(Compressor.prototype, {
                             right: candidate.value
                         });
                     }
-                    candidate.write_only = false;
+                    var assign = candidate;
+                    while (assign.write_only) {
+                        assign.write_only = false;
+                        if (!(assign instanceof AST_Assign)) break;
+                        assign = assign.right;
+                    }
                     return candidate;
                 }
                 // These node types have child nodes that execute sequentially,