pgsql: Fix integer underflow in shared memory debugging

Fix integer underflow in shared memory debugging

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

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/217928ddb45a8a3b03fc1f9d3d97166f1d9a9d12

Modified Files
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src/backend/utils/mmgr/dsa.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)