Fixed a bug in test_lockmode_for_conflict

In commit 8e65c1b, wrongly extracting offnum from
tuple(without checking null), so we were getting segment fault. Fixed
this.

Patch by Mahendra Singh Thalor, reviewed by Dilip Kumar

src/backend/access/zheap/zheapam.c

@@ -3540,11 +3540,14 @@ test_lockmode_for_conflict(Relation rel, Buffer buf, ZHeapTuple zhtup,
 		if (DoLockModesConflict(HWLOCKMODE_from_locktupmode(old_mode),
 								HWLOCKMODE_from_locktupmode(required_mode)))
 		{
-			OffsetNumber offnum = ItemPointerGetOffsetNumber(&zhtup->t_self);
-
 			*needwait = true;
 			if (subxid)
-				ZHeapTupleGetSubXid(buf, offnum, urec_ptr, subxid);
+			{
+				Assert(zhtup != NULL);
+				ZHeapTupleGetSubXid(buf,
+									ItemPointerGetOffsetNumber(&zhtup->t_self),
+									urec_ptr, subxid);
+			}
 		}
 
 		/*