Actually execute checks defined in base classes

[t184256]

Several base classes for crypto algorithms contain sanity checks that never get executed in children classes, which is misleading and could lead to inconsistent error reporting across implementations.

[t184256]

Example (AES, others have similar variety):

• A check on a plaintext length in decode: https://github.com/tomato42/tlslite-ng/blob/cef7d75f79fd746c001b339399257098a00b46be/tlslite/utils/aes.py#L30
• Invocation of this check from openssl implementation: https://github.com/tomato42/tlslite-ng/blob/cef7d75f79fd746c001b339399257098a00b46be/tlslite/utils/openssl_aes.py#L41
• A different style of invoking it, python implementation: https://github.com/tomato42/tlslite-ng/blob/cef7d75f79fd746c001b339399257098a00b46be/tlslite/utils/python_aes.py#L51
• Not invoking this check, pycrypto implentation: https://github.com/tomato42/tlslite-ng/blob/cef7d75f79fd746c001b339399257098a00b46be/tlslite/utils/pycrypto_aes.py#L27

Also, 3DES python implementation does not inherit from the base class at all.

[tomato42]

Given the test coverage from tlsfuzzer, it doesn't look like it's something visible/exploitable/inconsistent on TLS level, but it would be nice to fix it to reduce code duplication and unify behaviour between implementations. Mostly so that external users can depend on consistent behaviour of the decryption functions, irrespective of the selected concrete implementation.

[rajdroid]

@tomato42 if there is no deadline attached I can take a look at this request.

[tomato42]

@rajdroid there is no deadline, feel free to pick it up

[rajdroid]

@tomato42 please assign it to me then.