cgen: fix for in mut val in array.index() (#12410)

vlib/v/gen/c/array.v

@@ -762,6 +762,9 @@ fn (mut g Gen) gen_array_index(node ast.CallExpr) {
 	}
 	g.expr(node.left)
 	g.write(', ')
+	if node.args[0].expr.is_auto_deref_var() {
+		g.write('*')
+	}
 	g.expr(node.args[0].expr)
 	g.write(')')
 }

vlib/v/tests/for_in_mut_array_index_test.v

@@ -0,0 +1,12 @@
+fn test_for_in_mut_array_index() {
+	mut arr := [1, 2, 3, 4, 5]
+	mut rets := []int{}
+
+	for mut val in arr {
+		inx := arr.index(val)
+		println(inx)
+		rets << inx
+	}
+
+	assert rets == [0, 1, 2, 3, 4]
+}