unused-but-set-variable warning on REL_13_STABLE

[Nathan Bossart]

My compiler emits the following warning on REL_13_STABLE when assertions
are disabled:

        reorderbuffer.c:2471:8: warning: variable 'spilled' set but not used 
[-Wunused-but-set-variable]
         2471 |         Size            spilled = 0;
              |                         ^

Adding PG_USED_FOR_ASSERTS_ONLY (as in the attached patch) seems sufficient
to suppress this warning.  On newer versions, this variable is used for
more than assertions, so the patch only needs to be applied to v13.  Since
this is trivial, I plan to commit it shortly.

-- 
nathan

diff --git a/src/backend/replication/logical/reorderbuffer.c 
b/src/backend/replication/logical/reorderbuffer.c
index c7f8fa6216..56c25e3a6d 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -2468,7 +2468,7 @@ ReorderBufferSerializeTXN(ReorderBuffer *rb, 
ReorderBufferTXN *txn)
        dlist_mutable_iter change_i;
        int                     fd = -1;
        XLogSegNo       curOpenSegNo = 0;
-       Size            spilled = 0;
+       Size            spilled PG_USED_FOR_ASSERTS_ONLY = 0;
 
        elog(DEBUG2, "spill %u changes in XID %u to disk",
                 (uint32) txn->nentries_mem, txn->xid);