MockERC4626 Fix

[jalextowle]

Description

Ignores underflows in the mock yield sources.

Review Checklists

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[[Reviewer Name]]

• [ ] Tokens
  • [ ] Do all approve calls use forceApprove?
  • [ ] Do all transfer calls use safeTransfer?
  • [ ] Do all transferFrom calls use msg.sender as the from address?
    • [ ] If not, is the function access restricted to prevent unauthorized token spend?
• [ ] Low-level calls (call, delegatecall, staticcall, transfer, send)
  • [ ] Is the returned success boolean checked to handle failed calls?
  • [ ] If using delegatecall, are there strict access controls on the addresses that can be called? It shouldn't be possible to delegatecall arbitrary addresses, so the list of possible targets should either be immutable or tightly controlled by an admin.
• [ ] Reentrancy
  • [ ] Are functions that make external calls or transfer ether marked as nonReentrant?
    • [ ] If not, is there documentation that explains why reentrancy is not a concern or how it's mitigated?
• [ ] Gas Optimizations
  • [ ] Is the logic as simple as possible?
  • [ ] Are the storage values that are used repeatedly cached in stack or memory variables?
  • [ ] If loops are used, are there guards in place to avoid out-of-gas issues?
• [ ] Visibility
  • [ ] Are all payable functions restricted to avoid stuck ether?
• [ ] Math
  • [ ] Is all of the arithmetic checked or guarded by if-statements that will catch underflows?
  • [ ] If Safe functions are altered, are potential underflows and overflows caught so that a failure flag can be thrown?
  • [ ] Are all of the rounding directions clearly documented?
• [ ] Testing
  • [ ] Are there new or updated unit or integration tests?
  • [ ] Do the tests cover the happy paths?
  • [ ] Do the tests cover the unhappy paths?
  • [ ] Are there an adequate number of fuzz tests to ensure that we are covering the full input space?

[dpaiton]

This should fix the contract errors cited here: https://github.com/delvtech/hyperdrive-rs/issues/104