From Wikipedia:
For purely real inputs, X(k) = X(N-k)*; therefore you only need to give half the components (the other half are basically the same, but their complex conjugate).
So it is efficient not to compute and store them; if you really need these values, you use the above formula to compute them on the fly.
That this is true is hinted at by the statement (in the comments of the code you linked to above):
// The first two returned entries are the real
// parts of the first and last value from the conjugate symmetric
// output, which are necessarily real. The length must be a power
// of 2.