Handle zero length plaintext for VIA PadLock functions

Handle zero length plaintext for VIA PadLock functions

If the plaintext is shorter than the block size of the used cipher,
_gnutls_auth_cipher_encrypt2_tag calls _gnutls_cipher_encrypt2 with
textlen = 0. padlock_ecb_encrypt and padlock_cbc_encrypt assume that the
plaintext length (last parameter) is greater than zero and segfault
otherwise. The assembler code for both functions is automatically
generated and imported from OpenSSL, so to ease maintenance the length
should be validated in the functions that call padlock_ecb_encrypt or
padlock_cbc_encrypt.