dsa_dump would print a large negative number instead of zero for
segment bin 0. Fix by explicitly checking for underflow and add
special case for bin 0. Backpatch to all supported versions.
Author: Ian Ilyasov <ianilyasov@outlook.com> Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/GV1P251MB1004E0D09D117D3CECF9256ECD502@GV1P251MB1004.EURP251.PROD.OUTLOOK.COM
Backpatch-through: v12