heap-overflow in cryptopp_aes128_cbc

[etam]

cryptopp_aes128_cbc test is failing for me with "free(): invalid pointer" and valgrind reports "Invalid write of size 8" at CryptoPP::CBC_Decryption::ProcessData.

This commit ff6f963 was supposed to fix the issue, but I think it didn't change anything.

cipher.size() == 32 and string192.size() == 24 therefore decrypting cipher to string192 must overflow.

[chfast]

Confirmed in #5480.

I believe it was assumed that decryption will handle the padding.