ORCA: guard RetrieveExtStatsInfo against dropped statistics OID

When a DROP STATISTICS (or ALTER TABLE DROP COLUMN) drops a statistics
object within the same transaction, the relation's rd_statlist can still
contain a stale StatisticExtInfo entry whose backing pg_statistic_ext row
is already gone.  RetrieveExtStatsInfo iterated over that list and called
gpdb::GetExtStatsName(info->statOid) without checking for a NULL return.

GetExtStatsName returns NULL for a missing OID.  Passing NULL straight
into CDXLUtils::CreateDynamicStringFromCharArray triggers an assertion
failure in debug builds and undefined behaviour (corrupt memory) in
release builds, which in turn caused seemingly unrelated crashes later
in the same query — most visibly a SIGSEGV inside CMDKey::UlHashMDKey
called from Ptabdesc → RetrieveType when optimising an INSERT.

Fix: skip any StatisticExtInfo entry whose name cannot be resolved,
consistent with the same guard added to RetrieveExtStats in the
previous commit.

Author: yjhjstz

src/backend/gpopt/translate/CTranslatorRelcacheToDXL.cpp

@@ -573,9 +573,17 @@ CTranslatorRelcacheToDXL::RetrieveExtStatsInfo(CMemoryPool *mp, IMDId *mdid)
 			continue;
 		}
 
+		/* Guard against a stale rd_statlist entry whose backing pg_statistic_ext
+		 * row was already removed (e.g. DROP STATISTICS in the same transaction).
+		 * Skip silently so ORCA still sees the other valid statistics. */
+		char *raw_statname = gpdb::GetExtStatsName(info->statOid);
+		if (nullptr == raw_statname)
+		{
+			continue;
+		}
 		const CWStringConst *statname = GPOS_NEW(mp)
 			CWStringConst(CDXLUtils::CreateDynamicStringFromCharArray(
-							  mp, gpdb::GetExtStatsName(info->statOid))
+							  mp, raw_statname)
 							  ->GetBuffer());
 		CMDName *mdname = GPOS_NEW(mp) CMDName(mp, statname);