Add a quick test for get_thread_fast_rng()

src/test/test_crypto_rng.c

@@ -218,6 +218,14 @@ test_crypto_rng_fast(void *arg)
     tt_int_op(counts[i], OP_GT, 0);
   }
 
+  /* per-thread rand_fast shouldn't crash or leak. */
+  crypto_fast_rng_t *t_rng = get_thread_fast_rng();
+  for (int i = 0; i < N; ++i) {
+    uint64_t u64 = crypto_fast_rng_get_uint64(t_rng, UINT64_C(1)<<40);
+    tt_u64_op(u64, OP_GE, 0);
+    tt_u64_op(u64, OP_LT, UINT64_C(1)<<40);
+  }
+
  done:
   crypto_fast_rng_free(rng);
 }