net: hns3: set the freed pointers to NULL when lifetime is not end

driver inclusion
category: cleanup
bugzilla: https://gitee.com/openeuler/kernel/issues/I8PDAE


CVE: NA

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There are several pointers are freed but not setted to NULL,
and their lifetime is not end immediately. To avoid misusing
there wild pointers, set them to NULL.

Signed-off-by: Jian Shen <shenjian15@huawei.com>