checker: only cast as ast.Var if not unresolved, fixes #13561 (#13562)

vlib/v/checker/for.v

@@ -119,11 +119,13 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 					}
 					ast.SelectorExpr {
 						root_ident := node.cond.root_ident() or { node.cond.expr as ast.Ident }
+						if root_ident.kind != .unresolved {
 							if !(root_ident.obj as ast.Var).is_mut {
 								c.error('field `$node.cond.field_name` is immutable, it cannot be changed',
 									node.cond.pos)
 							}
 						}
+					}
 					else {}
 				}
 			}

vlib/v/checker/tests/for_in_invalid_identifier.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/for_in_invalid_identifier.vv:2:26: error: undefined ident: `s`
+    1 | fn main() {
+    2 |     for index, mut value in s.statements {
+      |                             ^
+    3 |         println('Hello $index $value')
+    4 |     }
+vlib/v/checker/tests/for_in_invalid_identifier.vv:3:26: error: no known default format for type `void`
+    1 | fn main() {
+    2 |     for index, mut value in s.statements {
+    3 |         println('Hello $index $value')
+      |                                ~~~~~
+    4 |     }
+    5 | }

vlib/v/checker/tests/for_in_invalid_identifier.vv

@@ -0,0 +1,5 @@
+fn main() {
+	for index, mut value in s.statements {
+		println('Hello $index $value')
+	}
+}