Currently: 1) libsodium and 2) nacl.
Note that the "nacl" variant is actually libsodium. We just want to make
sure the static analysers see the `VANILLA_NACL` code paths.
In this case, there was no way it would not be, but a code change down
the stack could cause a variable to become uninitialised. This avoids a
gcc warning and is more locally-correct.