mini-gmp mpz_powm incorrect result

[Guido Vranken]

The following program computes 1^0 % 1:

//#include <gmp.h>
#include "mini-gmp.c"
#include <stdio.h>

#define CF_CHECK_EQ(expr, res) if ( (expr) != (res) ) { goto end; }

int main(void)
{
    mpz_t a, b, c, res;
    char* s = NULL;
    /* noret */ mpz_init(a);
    /* noret */ mpz_init(b);
    /* noret */ mpz_init(c);
    /* noret */ mpz_init(res);

    CF_CHECK_EQ(mpz_set_str(a, "1", 10), 0);
    CF_CHECK_EQ(mpz_set_str(b, "0", 10), 0);
    CF_CHECK_EQ(mpz_set_str(c, "1", 10), 0);
    CF_CHECK_EQ(mpz_set_str(res, "0", 10), 0);

    /* noret */ mpz_powm(res, a, b, c);

    printf("%s\n", mpz_get_str(NULL, 10, res));
end:
    return 0;
}

The result should be 0, which is the case with regular libgmp, but with
mini-gmp the result is 1, and this is incorrect.

Tested on version 6.2.1 and the latest repository checkout, with recent
clang and gcc, on x64 Linux.