fix: use reference to catch exception

[levy5307]

What problem does this PR solve?

use reference to catch exception, to avoid build error.

Checklist

Tests

• Manual test (add detailed scripts or steps below)

1. send a dirty request by java client
2. and we can see there is a log about it

E2021-08-02 10:04:42.28 (1627869882028274338 6762) replica.replica0.0304000000000098: pegasus_server_write.cpp:115:on_batched_writes(): [2.4@{ip}:{port}] pegasus batch writes handler failed, from = {ip}:{port}, exception = no more data to read after end-of-buffer