this block of code is not reached on master since other than
initialisation, all other iterations have invalid_walk_tip
and to_mark_failed pointers in some form of this layout
where 1, 2, 3 and 4 are block heights.
invalid_walk_tip
↓
1 <- 2 <- 3 <- 4
↑
to_mark_failed
fix it so that blocks are correctly marked as BLOCK_FAILED_CHILD
if it's a descendant of BLOCK_FAILED_VALID block.
c99667583d