kdf: add missing null-terminator for self-test test-vector array
* cipher/kdf.c (selftest_pbkdf2): Add null-terminator to TV array.
This was causing kdf self-test to fail on s390x builds.
- GnuPG-bug-id: T5254
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>