Fix NullBufferBuilder::new_from_buffer wrong size assertion

[Kikkon]

Which issue does this PR close?

Closes #5445.

Rationale for this change

Avoid panic with the following code

        let mut builder = Int32Builder::new();
        for i in 0..64 {
            if i % 2 == 0 {
                builder.append_null();
            } else {
                builder.append_value(i as i32); 
            }
        }
        let array = builder.finish();
        let _ = array.into_builder().expect("into_builder");

What changes are included in this PR?

Are there any user-facing changes?

no

[tustvold]

Could we get a test please, e.g. the PR description

[Kikkon]

Could we get a test please, e.g. the PR description

Of course, I have already added the test.