eth/downloader: fix rare crash when parent header missing in db (#27945)

ReadSkeletonHeader can return nil if the header is missing, so we should
not access fields on it. Note that calling .Hash() on a nil header is fine, so there 
is no need to actually check for nil.

Co-authored-by: Martin Holst Swende <martin@swende.se>

eth/downloader/skeleton.go

@@ -648,7 +648,7 @@ func (s *skeleton) processNewHead(head *types.Header, final *types.Header, force
 	}
 	if parent := rawdb.ReadSkeletonHeader(s.db, number-1); parent.Hash() != head.ParentHash {
 		if force {
-			log.Warn("Beacon chain forked", "ancestor", parent.Number, "hash", parent.Hash(), "want", head.ParentHash)
+			log.Warn("Beacon chain forked", "ancestor", number-1, "hash", parent.Hash(), "want", head.ParentHash)
 		}
 		return true
 	}