Fix hoisting the var in ForIn

Close #913

lib/compress.js

@@ -1276,7 +1276,10 @@ merge(Compressor.prototype, {
                             var seq = node.to_assignments();
                             var p = tt.parent();
                             if (p instanceof AST_ForIn && p.init === node) {
-                                if (seq == null) return node.definitions[0].name;
+                                if (seq == null) {
+                                    var def = node.definitions[0].name;
+                                    return make_node(AST_SymbolRef, def, def);
+                                }
                                 return seq;
                             }
                             if (p instanceof AST_For && p.init === node) {

test/compress/issue-913.js

@@ -0,0 +1,20 @@
+keep_var_for_in: {
+    options = {
+        hoist_vars: true,
+        unused: true
+    };
+    input: {
+        (function(obj){
+            var foo = 5;
+            for (var i in obj)
+                return foo;
+        })();
+    }
+    expect: {
+        (function(obj){
+            var i, foo = 5;
+            for (i in obj)
+                return foo;
+        })();
+    }
+}