line-reader: Fix bad math when size_t is bigger than off_t.

When size_t is bigger than off_t, pos - r->length will end up very large,
instead of negative, when r->length > pos, so the MAX didn't help to get
rid of negative values.  I'm not sure that this should really happen in
practice, but it's better to avoid it.

Reported by GCC on x86-64.