misc/random: improve the fallback case

c365e2f7 added av_random_bytes() to avoid multiple
threads/processes getting the same seed. but the fallback case
still suffers from it.

also mix in a function pointer, which hopefully is randomized
per process due to ASLR (20+ years old technology) along with
the stack position which will be different on different threads.

misc/random.c

@@ -52,6 +52,8 @@ mp_rand_state mp_rand_seed(uint64_t seed)
             return ret;
         }
         seed = mp_raw_time_ns();
+        seed ^= (uintptr_t)&mp_rand_seed; // ASLR, hopefully
+        seed += (uintptr_t)&ret; // stack position
     }
 
     ret.v[0] = seed;