[bug]: HighestChanID should exclude zero-conf alias SCIDs

[Crypt-iQ]

The HighestChanID function is used to determine the highest SCID that we know of when syncing gossip with another peer. If genChanRangeQuery calls it outside of a historical query and a zero-conf SCID exists in the database, it should cause an underflow when the alias SCID height is subtracted from our best block. This would then get cast to a uint32 and would mean we send a very high number_of_blocks value to the peer.

lnd/discovery/syncer.go

Lines 925 to 932 in d9febbb

	startHeight = uint32(newestChan.BlockHeight - chanRangeQueryBuffer)
	}
	// Determine the number of blocks to request based on our best height.
	// We'll take into account any potential underflows and explicitly set
	// numBlocks to its minimum value of 1 if so.
	bestHeight := g.cfg.bestHeight()
	numBlocks := bestHeight - startHeight