"cut -f 2- A B" no longer triggers a double-free bug

"cut -f 2- A B" no longer triggers a double-free bug
* src/cut.c (cut_fields): Set file-scoped global to NULL after
freeing it.  This avoids a double-free (and core dump on some systems)
for this usage: "echo 1>a; echo 2>b; cut -f2- a b".  Reported by
James Hunt in <http://bugzilla.redhat.com/220312>.
* NEWS: List this bug fix.
* THANKS: Mention him.
* tests/misc/cut: New file.
* tests/misc/Makefile.am (TESTS): Add cut.