Fix packSamples calculating out-of-bounds

[kungfooman]

Fixes #4860

Hi @slimbuck, you suggested i < numSamples / 4 and then keep the * 4, but getting rid of all these divisions/multiplications via a simple + feels a bit too tempting, are you okay rewriting the for-loop like this?

To make sure the functionality didn't change, I wrote a little test (requires src/index.js changes tho to export packSamples):

const { data } = pc.packSamples([
    0.1, 0.2, 0.3, 0.4, // row 1
    0.5, 0.6, 0.7, 0.8,
    0.1, 0.2, 0.3, 0.4, // same row 1 for control
]);
console.table([
    [...data.slice( 0, 16)],
    [...data.slice(16, 32)],
    [...data.slice(32, 48)],
]);
data;

Output for with and without PR is the same:

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[slimbuck]

Yep looks good thanks!