Fix possible internal overflow in numeric division.

div_var_fast() postpones propagating carries in the same way as mul_var(),
so it has the same corner-case overflow risk we fixed in 246693e5,
namely that the size of the carries has to be accounted for when setting
the threshold for executing a carry propagation step.  We've not devised
a test case illustrating the brokenness, but the required fix seems clear
enough.  Like the previous fix, back-patch to all active branches.

Dean Rasheed