net: hns3: fix the exception when query imp info

mainline inclusion
from mainline-v5.15-rc2
commit 472430a7
category: bugfix
bugzilla: NA
CVE: NA

----------------------------

When the command for querying imp info is issued to the firmware,
if the firmware does not support the command, the returned value
of bd num is 0.
Add protection mechanism before alloc memory to prevent apply for
0-length memory.

Fixes: 0b198b0d ("net: hns3: refactor dump m7 info of debugfs")
Signed-off-by: Jiaran Zhang <zhangjiaran@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Reviewed-by: li yongxin <liyongxin1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>