Fix memory leak

[joerowell]

At the moment fpre.h leaks memory allocated in the constructor. Specifically, the constructor allocates memory like this:

for(int i = 0; i < THDS; ++i) {
      ...
      eq[i] = new Feq<T>(io[i], party);
      eq[THDS + i] = new Feq<T>(io2[i], party);
}

Whereas the destructor only frees the first half:

for(int i = 0; i < THDS; ++i) {
  ...
  delete eq[i];
}

This PR simply deletes the rest of the eq values. AddressSanitizer doesn't show any other leaks, so I think this is the only one.

[wangxiao1254]

Thanks!