eth/downloader: fix unexpected skeleton header deletion

[rjl493456442]

Fixes #26300

This PR fixes a scenario in which Geth will delete referenced skeleton header by mistake.

In the example listed in #26300

• Local skeleton header 325@0946fa
• New beacon header 327@e97873

These two headers are in different chain, reorg happens. Geth syncs up the missing skeleton
headers which links up with local chain. The link point is lower than 325.

Then there is a special logic in beacon sync that

// If there are only 2 subchains - the current one and an older
// one - and the old one consists of a single block, then it's
// the expected new sync cycle after some propagated blocks.

And if the associated header of leftover subchain is in range of
new subchain(just like the example), we delete the skeleton header
referenced by new subchain by mistake because we don't check
the hash at all.

Please check #26300 for more information.
Thanks @holiman for detailed analysis!

[karalabe]

Very nice catch @rjl493456442!

Added a followup commit to repro the case and trigger a similar crash without the fix. LGTM

[holiman]

LGTM