basenc: fix bug49741: using wrong decoding buffer length

basenc: fix bug49741: using wrong decoding buffer length

Emil Lundberg <lundberg.emil@gmail.com> reports in
https://bugs.gnu.org/49741 about a 'basenc --base64 -d' decoding bug.
The input buffer length was not divisible by 3, resulting in
decoding errors.

* NEWS: Mention fix.
* src/basenc.c (DEC_BLOCKSIZE): Change from 1024*5 to 4200 (35*3*5*8)
which is divisible by 3,4,5,8 - satisfying both base32 and base64;
Use compile-time verify() macro to enforce the above.
* tests/misc/basenc.pl: Add test.