ping^2: [patchv2] Fix 100x slowdown regression on DWZ files

[Doug Evans]

Doug Evans writes:
 > Do hashtables support calling htab_find_slot with INSERT but then
 > not assigning the slot a value if it was empty?
 > I could be wrong but I think it does.
 > If so, we can remove one call to htab_find_slot here.

I was wrong.
I thought I was, but I couldn't remember, and couldn't remember where
I had seen this.  But htab_find_slot_with_hash does assume
that if you call it with INSERT, and the slot isn't already used,
then you're going to fill in the slot.
No matter, it was just a thought.