Fix coverity defects: CID 150919, 150923

CID 150919: Buffer not null terminated (BUFFER_SIZE_WARNING)
CID 150923: Buffer not null terminated (BUFFER_SIZE_WARNING)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: luozhengzheng <luo.zhengzheng@zte.com.cn>
Closes #5298