bug: debugger should signal on completion to avoid overfetching when buffering close to the end

[goldfish088]

Describe the bug

To Reproduce

Use this C program and debug until around appending the 4th node, then click play to buffer next set of debug states.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

struct node {
    int val;
    struct node *next;
};

struct node *append(struct node *root, int val) {
    if (root == NULL) {
        struct node *ret = malloc(sizeof(struct node));
        ret->val = val;
        ret->next = NULL;
        return ret;
    }
    
    root->next = append(root->next, val);
    return root;
}

int main(void) {
    int N = 5;
    struct node *n = NULL;
    
    for (int i = 0; i < N; ++i) {
        n = append(n, i+1);
    }
}

Expected behavior

If buffering tries to fetch more debug steps than remaining in program execution, then it should not error lol

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.