Support `read(0)`

[mathiasbynens]

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Allowing .read(0) enables simpler code in cases like the following (which I just ran in to):

reader.read(0x2, function(byteCount, buffer) {
    // The next 2 bytes indicate the size of the string that follows.
    var descriptionLength = buffer.readUIntLE(0, byteCount);
    reader.read(descriptionLength, function(byteCount, buffer) {
        // Imagine lots of code here that reads the string.
        // The exact same code path must be taken if `descriptionLength == 0`,
        // but currently binary-reader doesn’t allow that and forces me to duplicate
        // code or to wrap everything in promises to avoid the sync/async difference.
        // This patch fixes all that by simply allowing `.read(0)`.
    });

Thanks for considering merging this. I really enjoy using this module in my hobby project!