Missing checking of allocation retvals

[est31]

The library isn't watertight regarding the checking of allocation results. The result should be sent to the caller like a nice C library should do.

• ignoring the result of hash_num may lead to read of uninit memory, and wrong results instead of errors. This point comes from minetest/minetest#3921 and was the motivation of the issue.
• similar in update_hash_n
• mpz_init in new_ng is called before checking the return value

[est31]

fixed by 16bffd8