Fix memory leak in hkdf_expand

[joerowell]

At the moment hkdf_expand allocates memory on each iteration of the loop but never frees it:

for(int i = 1; i <= N; i++) {
    auto input = new block[8 + info_len + cur_T_len];
    ...
}

This means that the program leaks memory each time hkdf_expand is called.

This PR fixes this by:

1. Moving the allocation outside of the loop. This should also make the code run faster.
2. Freeing the memory after the loop terminates.