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

Author: danielgustafsson

src/backend/utils/mmgr/dsa.c

@@ -1105,9 +1105,13 @@ dsa_dump(dsa_area *area)
 		{
 			dsa_segment_index segment_index;
 
-			fprintf(stderr,
-					"    segment bin %zu (at least %d contiguous pages free):\n",
-					i, 1 << (i - 1));
+			if (i == 0)
+				fprintf(stderr,
+						"    segment bin %zu (no contiguous free pages):\n", i);
+			else
+				fprintf(stderr,
+						"    segment bin %zu (at least %d contiguous pages free):\n",
+						i, 1 << (i - 1));
 			segment_index = area->control->segment_bins[i];
 			while (segment_index != DSA_SEGMENT_INDEX_NONE)
 			{