fix corner case in merge_vars (#4550)

fixes #4548

lib/compress.js

@@ -4979,7 +4979,9 @@ merge(Compressor.prototype, {
                     var marker = new TreeWalker(function(node) {
                         if (node instanceof AST_Destructured) return;
                         if (node instanceof AST_DefaultValue) {
+                            push();
                             node.value.walk(tw);
+                            pop();
                             node.name.walk(marker);
                         } else if (node instanceof AST_DestructuredKeyVal) {
                             if (node.key instanceof AST_Node) {

test/compress/default-values.js

@@ -1570,3 +1570,29 @@ issue_4540: {
     expect_stdout: "undefined"
     node_version: ">=6"
 }
+
+issue_4548: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        A = "foo";
+        var a = A;
+        [ b = c = "bar" ] = [ console, console.log(a) ];
+        console.log(c);
+        var c;
+    }
+    expect: {
+        A = "foo";
+        var a = A;
+        [ b = c = "bar" ] = [ console, console.log(a) ];
+        console.log(c);
+        var c;
+    }
+    expect_stdout: [
+        "foo",
+        "undefined",
+    ]
+    node_version: ">=6"
+}