two stack using array in c-hw to avoid insertion of zero while popping?

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C

Hi, I am trying to implement two stacks in one array.
The push operation of both the stacks works.
But in the popping, when i pop an element from the first stack then the remaining elements in second stack shift to left but a zero will be placed at the second stack's top. How to avoid this zero?
The code is ginven below.
Somebody please help.

#include
#include
#define max 9
int i,stk[max]={NULL},top1=-1,top2=-1,btn1=0,btn2=0;
void display()
{
  printf("\nThe stack is...\n\n");
  for(i=0;i<=top2;i++)
    printf("%d  ",stk[i]);
}
void push_first()
{
  if(top1!=max)
  {
    for(i=top2+1;i>=btn2;i--)
      stk[i]=stk[i-1];
    top2++;
    btn2++;
    top1++;
    printf("\nEnter the element:");
    scanf("%d",&stk[top1]);
  }
  display();
}
void push_second()
{
  if(top2!=max)
  {
    top2++;
    printf("\nEnter the element:");
    scanf("%d",&stk[top2]);
  }
  display();
}
void pop_first()
{
  if(top1!=btn1-1)
  {
    printf("\nPopped element :%d",stk[top1]);
    for(i=btn2-1;i<top2;i++)
    {
      stk[i]=stk[i+1];
      stk[i+1]='\0';
    }
    btn2--;
    top1--;
  }
  display();
}
void pop_second()
{
  if(top2!=top1)
  {
    printf("\nPopped element :%d",stk[top2]);
    top2--;
  }
  display();
}
int main()
{
  int ch;
  clrscr();
  do
  {
    printf("\n\n  MENU");
    printf("\n1.Push first\n2.Push second\n3.Pop first\n4.Pop second\n5.Display\n6.Exit");
    printf("\nEnter your choice:");
    scanf("%d",&ch);
    switch(ch)
    {
    case 1:push_first();
      break;
    case 2:push_second();
      break;
    case 3:pop_first();
      break;
    case 4:pop_second();
      break;
    case 5:display();break;
    case 6:exit(0);
    default:printf("\nInvalid choice!");
    }
  }while(ch!=6);
  getch();
  return(0);
}