dog: memory leak in trace_read_buffer

In function trace_read_buffer, the pointer 'buf' is allocated
via xmalloc, but its memory is not released when returns.

Signed-off-by: Meng An <anmeng.an@alibaba-inc.com>
Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>