[Wallet] Fix stake split output count calculation

[PeterL73]

Issue being fixed #1571

The UTXO value of a stake split is sometimes smaller then the stake split threshold
The help description for setstakesplitthreshold states:

This will set the stake-split threshold value.
Whenever a successful stake is found, the stake amount is split across as many outputs 
(each with a value higher than the threshold) as possible.

So you would not expect the value of the UTXOs to be lower then the threshold value
This happens because the number of stake split outputs is calculated including the masternode payment
An example (for testnet) with a UTXO with a value of 9992 and the stake split threshold set to 500:

9992 + 10 (total block reward: 4 stake reward + 6 masternode payment) = 10002
10002 / 500 = 20.004
(int) 20.004 = 20

However the staker only receives the stake reward (4) so the total amount after staking will be 9996 which is split across 20 UTXOs with a value of 499.8 each

What was done

The masternode payment is excluded from the stake split output count calculation

How Has This Been Tested

On testnet 2 UTXOs were made (9992 each) and the stake split threshold was set to 500
The expected outcome would be a split across 19 UTXOs with a value of 526.1 each

9992 + 4 (stake reward) = 9996
9996 / 500 = 19.992
(int) 19.992 = 19
9996 / 19 = 526.1

Before

On stake 20 UTXOs, with a value of 499.8 each, were created
https://testnet.rockdev.org/tx/b966ff04d35ecae596f0dfde3977862268995d63939af3ab90bb172b38c37f20

After

On stake 19 UTXOs, with a value of 526.1 each were created
https://testnet.rockdev.org/tx/e80c79ad1ac41aa096b4613462be348e7a1a4a70755c4943087a83be8865de7a

[panleone]

utACK:
Good catch and straightforward fix

[Liquid369]

tACK 6e51d38

Wish I has tested this abit sooner sorry for the delay!

[Duddino]

utACK

[Fuzzbawls]

ACK 6e51d38