rusty-gd: give special treament to unconstrained only-children

usually a bad thing with actual human kids :p

if a packet only has one child and that child is running around
unconstrained and causing mischief, assume we may be able to parse
remaining bytes as the child because it's the only possibility

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost DirectHciTest
Change-Id: I759d041c602e1d96eb640a2ec911bef2b80957bb