Fix possible internal overflow in numeric multiplication.
mul_var() postpones propagating carries until it risks overflow in its
internal digit array. However, the logic failed to account for the
possibility of overflow in the carry propagation step, allowing wrong
results to be generated in corner cases. We must slightly reduce the
when-to-propagate-carries threshold to avoid that.
Discovered and fixed by Dean Rasheed, with small adjustments by me.
This has been wrong since commit
d72f6c75038d8d37e64a29a04b911f728044d83b,
so back-patch to all supported branches.