refactor(buffer_worker): less defensive on inflight counter decrement

apps/emqx_resource/src/emqx_resource_buffer_worker.erl

@@ -1440,10 +1440,10 @@ ack_inflight(InflightTID, Ref, Id, Index) ->
             [] ->
                 0
         end,
+    ok = dec_inflight(InflightTID, Count),
     IsKnownRef = (Count > 0),
     case IsKnownRef of
         true ->
-            ok = dec_inflight(InflightTID, Count),
             emqx_resource_metrics:inflight_set(Id, Index, inflight_num_msgs(InflightTID));
         false ->
             ok
@@ -1466,15 +1466,16 @@ mark_inflight_items_as_retriable(Data, WorkerMRef) ->
     ok.
 
 %% used to update a batch after dropping expired individual queries.
-update_inflight_item(InflightTID, Ref, NewBatch, NumExpired) when NumExpired > 0 ->
+update_inflight_item(InflightTID, Ref, NewBatch, NumExpired) ->
     _ = ets:update_element(InflightTID, Ref, {?ITEM_IDX, NewBatch}),
-    ok = dec_inflight(InflightTID, NumExpired),
-    ok.
+    ok = dec_inflight(InflightTID, NumExpired).
 
 inc_inflight(InflightTID, Count) ->
     _ = ets:update_counter(InflightTID, ?SIZE_REF, {2, Count}),
     ok.
 
+dec_inflight(_InflightTID, 0) ->
+    ok;
 dec_inflight(InflightTID, Count) when Count > 0 ->
     _ = ets:update_counter(InflightTID, ?SIZE_REF, {2, -Count, 0, 0}),
     ok.