Repair array subscript overrun identified by Yichen Xie. Reduce the

Repair array subscript overrun identified by Yichen Xie.  Reduce the
value of MAX_TIME_PRECISION in floating-point-timestamp-storage case
from 13 to 10, which is as much as time_out is actually willing to print.
(The alternative of increasing the number of digits we are willing to
print looks risky; we might find ourselves printing roundoff garbage.)