mu-sexp-convert: correct etime->time_t conversion.

Of the two parameters in an elisp time object that are actually
needed, only the first---`(car t)`---was previously used, causing
inaccuracies of up to 2^16 seconds (i.e. 3/4ths of a day).

The second needed parameter---`(cdr (car t))`---is now properly
utilized.