Please explain why this is needed: the CBlockIndexWorkComparator is dereferencing the pointer, which causes the find to segfault

We have two choices:

1. Modify CBlockIndexWorkComparator to treat nullptr as least work
2. Not call the comparator in the first place (this patch)

I chose to not go with option 1 because I did not want to touch validation code. However making that change would mean less worrying about it in multiple call sites here, so it may be worth revisiting.

I'm OK with the patch, I just want you to add a comment to explain that the nullptr dereference issue is from the comparator and not the map, so that someone reading this code won't remove the check by mistake, as it's not obvious from the code.

Here it's IsWorthPolling that needs the proof to be non null