Nighly build from 2017-06-15 crashes (https://bitcoin.jonasschnelli.ch/build/176)
Nighy build from 2017-06-14 works fine (https://bitcoin.jonasschnelli.ch/build/175)

Relevant commit must be between 228c319a and fbf5d3ba.

```shell
jonasschnelli@Jonass-MBP:~/Documents/bitcoin/bitcoin$ ~/Downloads/bitcoin-0.14.n/bin/bitcoind --regtest
Abort trap: 6
jonasschnelli@Jonass-MBP:~/Documents/bitcoin/bitcoin$ lldb ~/Downloads/bitcoin-0.14.n/bin/bitcoind
(lldb) target create "/Users/jonasschnelli/Downloads/bitcoin-0.14.n/bin/bitcoind"
Current executable set to '/Users/jonasschnelli/Downloads/bitcoin-0.14.n/bin/bitcoind' (x86_64).
(lldb) run --regtest
Process 29763 launched: '/Users/jonasschnelli/Downloads/bitcoin-0.14.n/bin/bitcoind' (x86_64)
Process 29763 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
    frame #0: 0x00007fff94f74d42 libsystem_kernel.dylib`__pthread_kill + 10
libsystem_kernel.dylib`__pthread_kill:
->  0x7fff94f74d42 <+10>: jae    0x7fff94f74d4c            ; <+20>
    0x7fff94f74d44 <+12>: movq   %rax, %rdi
    0x7fff94f74d47 <+15>: jmp    0x7fff94f6dcaf            ; cerror_nocancel
    0x7fff94f74d4c <+20>: retq   
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x00007fff94f74d42 libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fff950625bf libsystem_pthread.dylib`pthread_kill + 90
    frame #2: 0x00007fff94eda4bb libsystem_c.dylib`__abort + 140
    frame #3: 0x00007fff94edad7e libsystem_c.dylib`__stack_chk_fail + 205
    frame #4: 0x0000000100270364 bitcoind`___lldb_unnamed_symbol4989$$bitcoind + 228
    frame #5: 0x0000000100046660 bitcoind`___lldb_unnamed_symbol475$$bitcoind + 32
    frame #6: 0x0000000100003be9 bitcoind`___lldb_unnamed_symbol1$$bitcoind + 2665
    frame #7: 0x00000001000042a1 bitcoind`___lldb_unnamed_symbol2$$bitcoind + 49
    frame #8: 0x00007fff94e46235 libdyld.dylib`start + 1
(lldb) 
```
{
    rng.SetKey(seed.begin(), 32);
}
bool Random_SanityCheck()
{
    uint64_t start = GetPerformanceCounter();
    /* This does not measure the quality of randomness, but it does test that
     * OSRandom() overwrites all 32 bytes of the output given a maximum
     * number of tries.
     */
    static const ssize_t MAX_TRIES = 1024;
    uint8_t data[NUM_OS_RANDOM_BYTES];
    bool overwritten[NUM_OS_RANDOM_BYTES] = {}; 
    int num_overwritten;
    int tries = 0;
    
    do {
        memset(data, 0, NUM_OS_RANDOM_BYTES);
        GetOSRand(data);
        for (int x=0; x < NUM_OS_RANDOM_BYTES; ++x) {
            overwritten[x] |= (data[x] != 0);
        }
        num_overwritten = 0;