Decoder checks SMS PDU ValidityPeriodFormat incorrectly

[ah9142]

Decoding an SMS-Submit PDU with ValidityPeriodFormat is unsupported and I expect it to return an error.
However, currently there is no error, but instead garbled / uninitialized data is returned.

Reproduction:
See new unit test PR: #36

Fix:
See PR: #36

[xlab]

Ty for the PR and tests!