Here, fdp points to the new fdtable, copied from that of the parent
process. There is a window after the fdtable is copied, and before
kqueue_fork_copy_knote() runs, where a thread in the parent could have
grown the parent's fdtable and registered a knote with ident larger than
the size of the child's fdtable.
Add a bounds check for this case; skip the knote if it is referencing a
non-existent file.