Cannot check for null getting error in an if statement

I have a system call for the Linux kernel. When checking for null in an if statement, the process terminates:

unable to handle kernel null pointer reference at [position]

The faulty code below:

struct task_struct *task;
int found = 0;
if ((task = find_task_by_vpid(pid))) // task was found when code ran
{
    found = 1;
}
if (found)
{
    char* nothing = "NULL";
    struct vm_area_struct *vma;
    ...
    vma = task->mm->mmap;
    while (vma != NULL)
    {
    ...
        if (!vma->vm_file->f_path.dentry->d_name.name)
        {
            printk("The file name mapped to this is %s\n", nothing);
        }
        ...
    }
    ...
    return 0;
}

Why does this not work?