this was mostly an inconvenience for error reporting, but fully broke
the debugger (because the debugger does *a lot* of eager position
resolution). copying the line offsets into a local and filling that
local when empty without also storing the calculated offsets back does
kind of ... not cache anything.
fixes #165
Change-Id: Iccb0ba193ce2f15c832978daecf7b9bebbbe8585