pp_formline: don't overgrow PL_formtarget

pp_formline: don't overgrow PL_formtarget

In various places, PL_formtarget is grown by fudge bytes.
But fudge is already equal to the whole width of the format line,
and PL_formtarget is pre-grown by fudge at the start, so normally
there's no need to extend it further. So don't.
Instead, only grow it by the amount needed (which will ormally be nothing)
as a safety measure.

Also add an assert at the end to check that we haven't overrun the buffer.