A newly appended block header should not build on an invalid chain

[yixiao5428]

This is a security vulnerability.

Currently, a new header descendant from an invalid block is still accepted.

A possible solution is to get the ancestor of the block, check the validity of the block indexes, and return error is failed, in the AcceptBlockHeader() function of src/main.cpp after line 3714.

Similar fix from Bitcoin: https://github.com/bitcoin/bitcoin/commit/015a5258adffb0cf394f387a95ac9c8afc34cfc3.

Reported by 6004ed5feaa31ae9df36b5dbc60f0fa53255a5fb734334082c6d202405fc738c.