Make sure that GetRandomBytes never fails

We're using GetRandomBytes in several contexts where it's either
unwieldy to return an error, or an error would mean a fatal exception
anyhow.

@gmaxwell checked OpenSSL a while ago and discovered that it never
actually fails, but it can't hurt to be a bit paranoid here.

src/random.h

@@ -19,7 +19,7 @@ void RandAddSeedPerfmon();
 /**
  * Functions to gather random data via the OpenSSL PRNG
  */
-bool GetRandBytes(unsigned char* buf, int num);
+void GetRandBytes(unsigned char* buf, int num);
 uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();