simplify impl of read table properties

[Connor1996]

Rudra complains about the potential double-free issue https://paper.seebug.org/1728/#05-panic-safety-double-free-rustsec-2021-0011, so simplify it to avoid using mem::forgot

// Warning (UnsafeDataflow:/ReadFlow): Potential unsafe dataflow issue in 
// `rocksdb::SstFileReader::read_table_properties::callback`
// -> src/rocksdb.rs:2330:9: 2338:10
extern "C" fn callback<F: FnOnce(&TableProperties)>(
            ctx: *mut c_void,
            ptr: *const crocksdb_ffi::DBTableProperties,
        ) {
            unsafe {
                let caller = ptr::read(ctx as *mut F);
                caller(TableProperties::from_ptr(ptr));
            }
        }

[BusyJay]

Is the pointer still valid when props is dropped?

[Connor1996]

yes, it's a shared ptr from TableReader. In the context of read_table_properties, TableReader is promised not to be dropped.

[Connor1996]

/test

[Connor1996]

/build