Shortly after reading my post, Willem Pinckaers pointed out that the reseed_system_prng function sets the global variable system_prng_bufpos to 32. This means that after the first invocation of this function, the for loop right after the reseed call in system_prng_gen_block never executes. Hence, the ANSI X9.31 PRNG code is completely non-functional.

http://rpw.sh/blog/2015/12/21/the-backdoored-backdoor/