pp_formline(): avoid buffer overrun

pp_formline(): avoid buffer overrun

RT #130703

My recent commit v5.25.9-77-g90c3aa0 attempted to simplify buffer growth
in pp_formline(), but missed the operators which append data to
PL_formtarget *without* doing 'goto append'. These ops either append a
fieldsize's worth of bytes, or a \n (FF_NEWLINE). So grow by fieldsize
whenever we fetch something new, and for each FF_NEWLINE.