license.bbclass: Bound beginline and endline in copy_license_files()

Ensure that begin_idx (i.e., beginline - 1) and end_idx (i.e.,
endline) are positive numbers in copy_license_files(). This makes sure
the same lines are copied as populate_lic_qa_checksum() uses when it
calculates the checksum. Before, beginline=0 would typically lead to
that no lines were copied at all.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>