pdl: Fix parsing of reserved fields

We did not advance correctly past the reserved field in the case where
the field is _not_ part of a chunk.

The change is reflected in the unit tests here. In addition, Henri
will add a test for this to the canonical tests.

Tag: #feature
Bug: 233340330
Test: atest --host libuwb_core_tests
Change-Id: I8784b439976509822ecd42d217fcf10d54abde5e