Fix bug where insert-file-contents closes a file twice.

Fix bug where insert-file-contents closes a file twice.

* fileio.c (close_file_unwind): Don't close if FD is negative;
this can happen when unwinding a zapped file descriptor.
(Finsert_file_contents): Unwind-protect the fd before the point marker,
in case Emacs runs out of memory between the two unwind-protects.
Don't trash errno when closing FD.
Zap the FD in the specpdl when closing it, instead of deferring
the removal of the unwind-protect; this fixes a bug where a child
function unwinds the stack past us.