cgen: fix struct init 0 fields & tests

vlib/v/gen/cgen.v

@@ -1587,7 +1587,7 @@ fn (g mut Gen) struct_init(it ast.StructInit) {
 			g.writeln('\t.$field_name = $zero,') // zer0')
 		}
 	}
-	if it.fields.len == 0 {
+	if it.fields.len == 0 && info.fields.len == 0 {
 		g.write('0')
 	}
 	g.write('}')

vlib/v/gen/tests/1.c

@@ -65,7 +65,7 @@ int main(int argc, char** argv) {
 	int g = ((int)(3.0));
 	byte* bytes = ((byte*)(0));
     User* user_ptr = (User*)memdup(&(User){	.age = 0,
-0}, sizeof(User));
+}, sizeof(User));
 	return 0;
 }
 
@@ -88,7 +88,8 @@ i < 10; i++) {
 	});
 	array_User users = new_array_from_c_array(1, 1, sizeof(User), (User[]){
 		(User){
-			0},
+			.age = 0,
+		},
 	});
 	bool b = (*(bool*)array_get(bools, 0));
 	array_string mystrings = new_array_from_c_array(2, 2, sizeof(string), (string[]){

vlib/v/gen/tests/2.c

@@ -30,7 +30,8 @@ void init_user() {
 
 User get_user() {
 	User user = (User){
-	0};
+		.name = tos3(""),
+	};
 	return user;
 }
 

vlib/v/gen/tests/3.c

@@ -66,7 +66,9 @@ int main(int argc, char** argv) {
 	_vinit();
 	os__args = os__init_os_args(argc, (byteptr*)argv);
 	User user = (User){
-0};
+		.age = 0,
+		.name = tos3(""),
+	};
 	user.age = 10;
 	user.age++;
 	user.name = tos3("bob");