Merge bitcoindevkit/bdk#1731: fix(core): Fix checkpoint Drop stack overflow

2df5861c5e1bf308f1b215b19e00d882aee07e87 test(core): test `Drop` implementation of `CPInner` (valued mammal)
67e1dc37bff195d91f2959f3f4bf5d5d10da597c fix(core): Fix checkpoint Drop stack overflow (LLFourn)

Pull request description:

  Fixes #1634

  This needs a test that demonstrates the issue is fixed. I was hoping @ValuedMammal could do that for me.

ACKs for top commit:
  ValuedMammal:
    self-ACK 2df5861c5e1bf308f1b215b19e00d882aee07e87
  notmandatory:
    ACK 2df5861c5e1bf308f1b215b19e00d882aee07e87

Tree-SHA512: a431cb93505cbde2a9287de09d5faac003b8dfa01342cd22c6ca197d70a73948f94f55dfa365cc06b5be36f78458ed34d4ef5fa8c9e5e2989a21c7ce5b55d9ca