js/binary/utils.js: Fix jspb.utils.joinUnsignedDecimalString to work with negative bitsLow and low but non-zero bitsHigh parameter.

[reltuk]

We noticed that our generated javascript message implementations decode certain number fields incorrectly for some values when the fields have jstype = JS_STRING. I tracked it down to readZigzagVarint64String(), which does not always decode a field correctly. In particular, it returns the wrong result when it calls into jspb.utils.joinUnsignedDecimalString with a bitsLow value that is negative and a bitsHigh value that is less than 0x1FFFFF. None of the existing test cases appear to cover that case.

This PR changes joinUnsignedDecimalString to call jspb.utils.joinUint64 on its fast path instead, which gets us the right result.

It's also possible to change the line to

return '' + (jspb.BinaryConstants.TWO_TO_32 * bitsHigh + (bitsLow >>> 0));

or similar instead.

This is my first contact with this code and it wasn't clear to me if the bug is actually in the caller of joinUnsignedDecimalString. If joinUnsignedDecimalString is expecting purely unsigned parameters then it's possible that this isn't the desired fix. But it would be great if we could get a fix landed :).

Thanks for the awesome project which we make great use of throughout our stack :).

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[reltuk]

@googlebot I signed it!

[perezd]

LGTM, thank you for your contribution!