cgen: fix function generated code when returning from match

[felipensp]

Fix #20260 (it requires fix for #20169 to be applied to work the test case below)

import x.json2

fn maybe_map_map[T, U, X, Y](m map[T]U, f fn(T, U) !(X, Y) ) !map[X]Y {
	mut r := map[X]Y{}
	for k, v in m {
		nk, nv := f(k, v)!
		r[nk] = nv
	}
	return r
}
type Snowflake = int
struct User {
	id Snowflake
	name string
}
fn User.parse(j json2.Any) !User {
	match j {
		map[string]json2.Any {
			return User{
				id: j['id']! as int
				name: j['name']! as string
			}
		}
		else {
			return error('expected user to be object, got ${j.type_name()}')
		}
	}
}
struct Resolved {
	users ?map[Snowflake]User
}
fn Resolved.parse(j json2.Any) !Resolved {
	match j {
		map[string]json2.Any {
			return Resolved{
				users: if m := j['users'] {
					maybe_map_map[string, json2.Any, Snowflake, User](m as map[string]json2.Any, fn (k string, v json2.Any) !(Snowflake, User) {
						return Snowflake(k.int()), User.parse(v)!
					})!
				} else {
					none
				}
			}
		}
		else {
			panic('asdadsd')
		}
	}
}
fn main() {
	println('asd')
	Resolved.parse({
		'0': json2.Any('1')
		'2': '3'
		'a4': 5
		'6': '7'
	}) or {
		assert err.msg() == 'expected user to be object, got string'
		return
	}
	assert false, 'It should return error'
}

[spytheman]

Excellent work.