the current x_realloc() does:

void *x_realloc(void *ptr, size_t size)
{
    void *p2;
    if (!ptr) return x_malloc(size);
    p2 = malloc(size);
    if (!p2) {
        fatal("out of memory in x_realloc");
    }
    if (ptr) {
        memcpy(p2, ptr, size);
        free(ptr);
    }
    return p2;
}

there are multiple issues here:
 - the old pointer "ptr" has "size" bytes read from it, but there's no guarantee that the old buffer is larger than the new size
 - the second "if (ptr)" check is useless ... it already did a "if (!ptr)" at the top

since the first issue cant be fixed without tagging every alloc with custom information, the easiest answer is to do:
void *x_realloc(void *ptr, size_t size)
{
    void *p2 = realloc(ptr, size);
    if (!p2)
        fatal("out of memory in x_realloc");
    return p2;
}