pdl: Call conforms before attempting to parse

The ‘parse_inner’ method should only be called if the payload actually
conforms to the expected size of the packet. We might be able to make
this API prettier in the future to avoid this danger.

Tag: #feature
Bug: 274623973
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: I4144f50cbc7b55a97bc10ef527db654218c63173