ch4中os/src/mm/page_table.rs内find_pte函数需要调换2个if块顺序避免潜在BUG

os/mm/page_table.rs中的find_pte函数，如果参数是一个没有被映射过的VPN，函数也会返回PTE，调用该函数的translate函数（page_table.rs内）没有检查PTE合法的手段，原OS代码不报错是因为所查询的PPN都在合法范围内。
原代码：

fn find_pte(&self, vpn: VirtPageNum) -> Option<&mut PageTableEntry> {
        let idxs = vpn.indexes();
        let mut ppn = self.root_ppn;
        let mut result: Option<&mut PageTableEntry> = None;
        for (i, idx) in idxs.iter().enumerate() {
            let pte = &mut ppn.get_pte_array()[*idx];
            if i == 2 {
                result = Some(pte);
                break;
            }
            if !pte.is_valid() {
                return None;
            }
            ppn = pte.ppn();
        }
        result
    }

修改后：

fn find_pte(&self, vpn: VirtPageNum) -> Option<&mut PageTableEntry> {
        let idxs = vpn.indexes();
        let mut ppn = self.root_ppn;
        let mut result: Option<&mut PageTableEntry> = None;
        for (i, idx) in idxs.iter().enumerate() {
            let pte = &mut ppn.get_pte_array()[*idx];
            if !pte.is_valid() {
                return None;
            }
            if i == 2 {
                result = Some(pte);
                break;
            }
            ppn = pte.ppn();
        }
        result
    }

Open os/mm/page_table.rs and inspect find_pte, then check how translate uses its result. Ensure the validity check occurs before accepting the final page-table entry, and verify that an unmapped VPN returns None while a mapped VPN still resolves normally.