mpz_gcdext bug in mini-gmp

[Syed Jafri]

Hello GMP Team,

Thank you for the wonderful library, I have been using mini-gmp and I have run into a bug with the mpz_gcdext function. The full GMP version gives the correct output, while the mini-gmp version of the function fails at line 2911 of mini-gmp.c, at the code ```assert (mpz_even_p (t0) && mpz_even_p (s0));```

I have created a full reproduction with both mini-gmp and gmp, using the mpz_gcdext function and sample inputs at https://github.com/jafri/gmp-bug

I am currently using unmodified GMP and mini-GMP version 6.2.0, with Apple clang version 11.0. Please let me know if you need any additional information, I will provide it to you promptly. Looking forward to solving this bug as quickly as possible.

Thank you,
Syed Jafri