Ensure the first random number is non-zero when fuzzing

In fuzzing mode, isc_random uses a fixed seed for reproducibility. The particular seed chosen happened to produce zero as its first number, however commit bd251de0 introduced an initialization check in random_test that required it to be non-zero. This change adjusts the seed to avoid spurious test failures.

Also, remove the temporary variable that was used for initialization because it did not match the type of the thread-local seed array.