Merge bitcoin-core#516: improvements to random seed in src/tests.c

be40c4d Fixup for C90 mixed declarations. (Gregory Maxwell)
8b3841c fix bug in fread() failure check (Don Viszneki)
cddef0c tests: add warning message when /dev/urandom fails (Don Viszneki)

Pull request description:

  I've made two small changes to `src/tests.c` circa random seed generation.

  Added a warning when `/dev/urandom` fails, mostly to defend against the case that someone should use the code verbatim, but also to enhance its illustrative power.

  Also I fixed a bug with how the return value of `fread()` was being evaluated. In fact, `/dev/urandom` was never being applied before as the check on the return value of `fread()` always failed!

Tree-SHA512: 239dbe8316220c2f0e5b370bf9a18f78196e96cc4a7edea58cf2521b2c9cbc8da065be96aa859f90324d57e388d30f7670ce6bc1cca52e5162e5ca66b1a55b34

Author: gmaxwell

src/tests.c

@@ -4994,8 +4994,9 @@ int main(int argc, char **argv) {
         }
     } else {
         FILE *frand = fopen("/dev/urandom", "r");
-        if ((frand == NULL) || fread(&seed16, sizeof(seed16), 1, frand) != sizeof(seed16)) {
+        if ((frand == NULL) || fread(&seed16, 1, sizeof(seed16), frand) != sizeof(seed16)) {
             uint64_t t = time(NULL) * (uint64_t)1337;
+            fprintf(stderr, "WARNING: could not read 16 bytes from /dev/urandom; falling back to insecure PRNG\n");
             seed16[0] ^= t;
             seed16[1] ^= t >> 8;
             seed16[2] ^= t >> 16;