C++17 compatibility: unreachable `return`.

Clang rejects the `return` as unreachable in C++17 mode, but complains
about falling off the end of a function in C++14 mode. Add UNREACHABLE
to make it happy either way.

Bug: http://b/111067277
Test: builds
Change-Id: I119a806f41e88752976d6fd3638146ec6e08d620
1 file changed