[backport#13657] wallet: assert to ensure accuracy of CMerkleTx::GetBlocksToMaturity

Summary:
93de2891fa9cb8314573ba3a6ab764bc9c52444d wallet: assert to ensure accuracy of CMerkleTx::GetBlocksToMaturity (Ben Woosley)

Pull request description:

According to my understanding, it should not be possible for coinbase
transactions to be conflicting, thus it should not be possible for
GetDepthInMainChain to return a negative result. If it did, this would
also result in innacurate results for GetBlocksToMaturity due to the
math therein. asserting ensures accuracy.

https://github.com/bitcoin/bitcoin/pull/13657/commits/93de2891fa9cb8314573ba3a6ab764bc9c52444d

Backport of Core PR13657

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7026