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.
Backport of Core PR13657