misc/random: set constant seed for fuzzing

Fuzzing expect deterministic behavior. While we don't use this random
generator for anything significant, still good to set the seed, in case
of future usage.

misc/random.c

@@ -41,6 +41,9 @@ static inline uint64_t splitmix64(uint64_t *const x)
 
 void mp_rand_seed(uint64_t seed)
 {
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+    seed = 42;
+#endif
     mp_mutex_lock(&state_mutex);
     state[0] = seed;
     for (int i = 1; i < 4; i++)