libarchive/{b64,uu}: Improve atol8 checks

libarchive/{b64,uu}: Improve atol8 checks

Make sure that supplied values cannot overflow int64_t, which would be
an undefined behavior. Also do not accept empty strings as numbers.

Last but not least, check values before casting them to target data
types to avoid truncations.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>