I've solved the problem:

Code:
struct niz *bform(struct niz *koef, int n, int a)
{
        int i=0;
       struct niz *p=koef;
       struct niz *temp1=p;
       struct niz *proba=NULL;
       struct niz *temp2=p->pok;
       double b=temp1->koef;
       insertn(proba,b);
       for(i=1; i++; i=n-1)
       {
          temp1=temp1->pok;
          temp2=temp2->pok;
          b=a*temp1->koef+temp2->koef;
          insertn(proba, b);
       }
       return(proba);
}
Now I have runtime error 0xC0000005: Access Violation on line: b=a*temp1->koef+temp2->koef;. I've googled and found out it's due to program trying to access memory not reserved for it.
So, how can I fix this? Thanks.