userswap: fix NULL pointer dereference in uswap_unmap_anon_page()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM



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If old_pte is NULL, *old_pte will result in a null pointer dereference.
Fix this by adding a NULL check for old_pte.

Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>