Bad String Concat in C

str_concat.c

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


struct ByteSlice { uint8_t* ptr; uintptr_t len; };

/* caution this removes the null terminated char! */
struct ByteSlice nullTerminated_slice(uint8_t* str) {
    uintptr_t len = 0;
    if (str != 0) for (uint8_t* cursor = str; *cursor != 0; cursor+=1) {
        len+=1;
    }

    return (struct ByteSlice){ .ptr = str, .len = len };
}

struct MyString
{
    struct ByteSlice slice ;
    uintptr_t capacity; 
};

struct ByteSlice null_byte = { .ptr = "", .len = 1 };

struct ByteSlice myString_as_slice( struct MyString* string ) {
    return string->slice;
}
uint8_t* byteSlice_end(struct ByteSlice* slice) {
    return slice->ptr + slice->len;
}

struct MyString concat_strs ( struct ByteSlice* start, uintptr_t len )
{
    uintptr_t total_len = 0;
    struct ByteSlice* end = start + len;
    for ( struct ByteSlice* cursor = start; cursor != end; cursor+=1){
        total_len += cursor->len;
    }

    uint8_t* ptr = malloc(total_len);
    uint8_t* out_cursor = ptr;


    for ( struct ByteSlice* cursor = start; cursor != end; cursor+=1) {
        uint8_t * slice_end = byteSlice_end(cursor);
        for (uint8_t* slice_cursor = cursor->ptr; slice_cursor != slice_end; slice_cursor+=1){
            *out_cursor = *slice_cursor;
            out_cursor+=1;
        }
    }
    
    struct ByteSlice out_slice = { .ptr = ptr, .len = total_len };

    return (struct MyString){ .slice = out_slice, .capacity = total_len };  
}

int main() {
    uint8_t*  ab= "ab";
    uint8_t*  cd= "cd";
    uint8_t*  ef= "ef";

    struct ByteSlice slices[5] = { 
        nullTerminated_slice(ab),
        nullTerminated_slice(cd),
        nullTerminated_slice(ef),
        nullTerminated_slice("ghij"),
        null_byte,
    };

    struct MyString front = concat_strs(slices, 4);
    
    printf("%s\n", front.slice.ptr);

    front.slice.len-=1; // remove null byte

    struct ByteSlice more[3] = {
        myString_as_slice(&front),
        nullTerminated_slice("xyz"),
        null_byte,
    };

    struct MyString final = concat_strs(more, 3);

    printf("%s\n", final.slice.ptr);

    return 0;
}