Add default case to main decoding loop, improves x86+Conan(ARM)performance.
Add default case to main decoding loop, improves x86+Conan(ARM)performance.

Existing code does not have a default case to the switch.  This causes the compiler to emit code handling erroneous cases (`kImplicit` not cleared) on the hot path.

PiperOrigin-RevId: 484048924

google/riegeli,google/riegeli,google/riegeli,google/riegeli