fix(bindings/C): fix the memory found in valgrind.

[xyjixyjixyji]

This PR fix the memory leak found in the tests in #2653.

I believe the memory leak is not completely fixed and it could be bettered through more tests on the C binding functionalities.

[xyjixyjixyji]

There is still an invalid read problem in the list test.

[xyjixyjixyji]

I think that the problem originates from the opendal_list_entry_path()

(*self.inner).path().as_ptr() as *const c_char

The path() returns a &str, which is a reference to the String typed path. And we are taking the pointer of that String directly. I am doing this because I don't want to wrap a opendal_string type, that seems overly verbose...... When I implemented that, I didn't think that was an issue since as long as the user calls opendal_list_entry_path(), the Entry must be alive and the String must be on heap.

I think the problem is that Rust String is not nul-terminated in memory..... Since valgrind reports this, I think we'd better fix this.

Now its fixed.

[Xuanwo]

Thanks a lot!