Problem removing trailing whitespace from a char* pointer

I have a problem with this function. I am trying to remove trailing whitespace from a pointer of type char*.

Something like that:

char *ptr = "Soy un puntero dinámico y me sobran espacios al final       ";

With the difference that this one is static and the one I am handling is dynamic.

I was doing this function for it but for some reason that I can't see, the while is not working for me, that is to say that the value of c is c=0; when I should be older.

char *quita_espacios_al_final(char **s)
{
    char *p = *s;
    if(p==NULL)return NULL;

    unsigned int i=strlen(p);
    unsigned int k=i;  


    unsigned int c=0;

    while( *(p+i)==' ' &&  i>0)
    {           
        i--;
        c++;
    }

    char *tmp = new char [k-c+1];

    for(unsigned int j=0; j<k-c; j++)
        *(tmp+j)=*(p+j);        

    delete [] p;
    return tmp;
}

Any ideas why it doesn't work? o Another alternative to count whitespace at the end of the string? PS: I don't care if it's in C or C++ as long as it works, but I can't change the type, it has to be (char*).