fix corner case in side_effects (#4513)

fixes #4512

lib/compress.js

@@ -8073,15 +8073,7 @@ merge(Compressor.prototype, {
             }
             if (argname instanceof AST_Destructured) {
                 has_destructured = true;
-                var abort = false;
-                argname.walk(new TreeWalker(function(node) {
-                    if (abort) return true;
-                    if (node instanceof AST_DestructuredKeyVal) {
-                        var key = node.key;
-                        if (key instanceof AST_Node && has_arg_refs(key)) return abort = true;
-                    }
-                }));
-                if (abort) return false;
+                if (has_arg_refs(argname)) return false;
             }
             return true;
         });

test/compress/destructured.js

@@ -2384,3 +2384,17 @@ issue_4508: {
     ]
     node_version: ">=6"
 }
+
+issue_4512: {
+    options = {
+        side_effects: true,
+    }
+    input: {
+        console.log(function([ a, b = a ]) {}([]));
+    }
+    expect: {
+        console.log(function([ a, b = a ]) {}([]));
+    }
+    expect_stdout: "undefined"
+    node_version: ">=6"
+}