cgen: fix struct initialization for nested option struct (fix #17524) (#17525)

vlib/v/gen/c/struct.v

@@ -323,7 +323,12 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 					typ: field.typ
 				}
 				g.write('.${field_name} = ')
-				g.struct_init(default_init)
+				if field.typ.has_flag(.option) {
+					tmp_var := g.new_tmp_var()
+					g.expr_with_tmp_var(default_init, field.typ, field.typ, tmp_var)
+				} else {
+					g.struct_init(default_init)
+				}
 				return true
 			}
 		}

vlib/v/tests/option_struct_init_test.v

@@ -1,22 +1,20 @@
-struct Foo {
-	a int
-	b int
-	c int
+pub struct MyStruct {
+pub mut:
+	valueb ?int
 }
 
-struct Holder {
-mut:
-	i int
+pub struct MyStruct2 {
+pub mut:
+	valuea int
+	valueb ?MyStruct
 }
 
-fn add(mut h Holder) ?int {
-	h.i++
-	return h.i
-}
-
-fn test_struct_init_with_multiple_options() {
-	mut h := Holder{}
-	foo := Foo{add(mut h) or { 0 }, add(mut h) or { 0 }, add(mut h) or { 0 }}
-
-	assert foo == Foo{1, 2, 3}
+fn test_main() {
+	a := MyStruct2{
+		valuea: 1
+	}
+	assert a.str() == 'MyStruct2{
+    valuea: 1
+    valueb: Option(error: none)
+}'
 }