Fix compat layer ASN1_TIME_diff to accept NULL output params

[embhorn]

Description

The first two parameters to ASN1_TIME_diff() are int *pday and int *psec. These refer to the locations where output values of days and seconds are to be written. OpenSSL 1.0.2 and 1.1.1 accept NULL values for either pday, psec or both, always returning success/1 in these cases.

Fix in the equivalent wolfSSL function, wolfSSL_ASN_TIME_diff() to not require pday or psec to be non-NULL.

Fixes zd19346

Testing

Updated unit tests

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation