Issue 46 - Return 0 if sub would underflow

[ethereumdegen]

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[ethereumdegen]

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Fixed the underflow issue. But the excess repaid amount will not be withdrawable by the other lenders. This should be ok though if the lenders are only being guarenteed their deposited amount. One thing to note is that this amount will still be borrowable. And if liquidation of it occurs and goes negative, PoolTotalEstimatedValue can go to negative (underflow is handled) and this amount would be taken from the newer depositors rather than the actually available extra amount