Add missed bms_copy() in perform_pruning_combine_step

We were initializing a BMS to merely reference an existing one, which
would cause a double-free (and a crash) when the recursive algorithm
tried to intersect it with an empty one.  Fix it by creating a copy at
initialization time.

Reported-by: sqlsmith (by way of Andreas Seltenreich)
Author: Amit Langote
Discussion: https://postgr.es/m/87in923lyw.fsf@ansel.ydns.eu

Author: alvherre

src/backend/partitioning/partprune.c

@@ -2923,7 +2923,8 @@ perform_pruning_combine_step(PartitionPruneContext *context,
 				if (firststep)
 				{
 					/* Copy step's result the first time. */
-					result->bound_offsets = step_result->bound_offsets;
+					result->bound_offsets =
+						bms_copy(step_result->bound_offsets);
 					result->scan_null = step_result->scan_null;
 					result->scan_default = step_result->scan_default;
 					firststep = false;