Fix unsafe reader get in MessageDef

[yunhanw-google]

So.. this is completely unsafe. It will do weird and broken things if the value in the TLV is out of range for type T.

Originally posted by @bzbarsky-apple in #3491 (comment)

[turon]

Referencing this code:

[src/app/MessageDef.cpp]

    VerifyOrExit(aTLVType == reader.GetType(), err = CHIP_ERROR_WRONG_TLV_TYPE);

    err = reader.Get(*apLValue);

But that file has since been significantly refactored into an entire directory: src/app/MessageDef.

Difficult to say whether the issue still exists, but need to follow-up and verify for v1.

Removing v1_triage_split_4 tag.

[bzbarsky-apple]

The important part is that we fixed Get on the reader to not do broken things in #9944. So this is good now.