hh: verify uuid is 16 bit before calling As16Bit

The documentation explicitly says to call Is16Bit before calling
As16Bit. Apparently it's possible to crash the process when we skip
calling Is16Bit.

Bug: 316784874
Test: m -j
Flag: EXEMPT, additional checks, no logical change.
Change-Id: I4f871522524426b3da9bae88ab1a3fdcc0eed674