fix overflow in balance_to_point conversion

[SCJangra]

Closes #416

As I mentioned in the above issue balance_to_points conversion currently overflows the Balance types even before computing the actual points for the given tokens. This fixes that,

[cla-bot-2021]

User @SCJangra, please sign the CLA here.

[gpestana]

Thanks for the PR, it looks like a fix for this issue. One improvement could be adding some tests that ensure that the conversion is working as expected and the bounds are safe and that would fail with the code before the PR, wdyt?

As a suggestion, something along these lines:

        #[test]
	fn points_and_balance_conversions_are_safe() {
		ExtBuilder::default().build_and_execute(|| {
			let bonded_pool = BondedPool::<Runtime> {
				id: 123123,
				inner: BondedPoolInner {
					commission: Commission::default(),
					member_counter: 1,
					points: u128::MAX,
					roles: DEFAULT_ROLES,
					state: PoolState::Open,
				},
			};
			StakingMock::set_bonded_balance(bonded_pool.bonded_account(), u128::MAX);

			// max out the points and balance of the pool and make sure the conversion works as
			// expected ans does not overflow.
			assert_eq!(bonded_pool.balance_to_point(u128::MAX), u128::MAX);
			assert_eq!(bonded_pool.points_to_balance(u128::MAX), u128::MAX);
		})
	}

[SCJangra]

@gpestana I've shamelessly copied the code you posted into tests, I can't think of any more tests specifically for this issue. Let me know if you need anything else changed/added.

[gpestana]

Great, thanks!