Hopefully fix issue with converting auxattr with bit 31 set to/from JS value:

Poll with results "closed" (only visible to pollster until the poll is closed)
have bit 31 set in the auxattr message header field. Reportedly (by Nightfox)
this causes Error: can't convert 2147483648 to an integer when attempting
to use this message in JS.
I could'vd sworn I've solved this problem before, but maybe that was with
JS-C v1.5 - anyway, first convert to double using JS_ValueToNumber and then
typecast to uint32.
We probably will need to do something similar with the time values in year
2038 (holding my breath).