use simpler check in blocksync reactor

internal/blocksync/reactor.go

@@ -557,7 +557,7 @@ func (r *Reactor) poolRoutine(ctx context.Context, stateSynced bool, blockSyncCh
 				// validate the block before we persist it
 				err = r.blockExec.ValidateBlock(ctx, state, first)
 			}
-			if err == nil && state.ConsensusParams.Vote.RequireExtensions(state.LastBlockHeight+1) {
+			if err == nil && state.ConsensusParams.Vote.RequireExtensions(extCommit.Height) {
 				// if vote extensions were required at this height, ensure they exist.
 				err = extCommit.EnsureExtensions()
 			}