Fix continuation handling in add_lines_at
Fix continuation handling in add_lines_at

OSBS-7352

Signed-off-by: Tim Waugh <fe049355006bf7fdd11c5e47ffe7a5032af30704@redhat.com>

DBuildService/dockerfile-parse,jpopelka/dockerfile-parse,DBuildService/dockerfile-parse,jpopelka/dockerfile-parse