zdb: Fix asize overflow in verify_livelist_allocs()

Spacemap entry might be too big to fit into a block pointer ashift.
We hit an assertion trying to run zdb -bvy on a large pool. But
it seems the code does not really need size there, since we only
need to search for a range of offsets, so setting it to zero should
just make btree return position just before the first entry. I
suspect the previous code could actually miss the first entry
due to this if its size was smaller.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17764