multiprogress.c: Fix `split_string' to overestimate.
Doing the right thing in `advance_measure' is much harder if
`split_string' sometimes falls short of the requested bound because of
wide characters. On the other hand, overestimating is a little tricky
because we must remember to include all of the following zero-
width (e.g., combining) characters or the prefix will just be wrong.
But it's better to do this than come up with some kludge to make
`advance_measure' sometimes advance the state even when we've not
actually reached the right position.
~mdw / dvdrip / commit