fix execve()

Arrgghhh...

Sorry about that, I'd been sure I'd folded that one, but it actually got
lost.  Please apply - that unbreaks execve().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>

kernel/fork.c

@@ -1787,7 +1787,7 @@ bad_unshare_out:
 int unshare_files(struct files_struct **displaced)
 {
 	struct task_struct *task = current;
-	struct files_struct *copy;
+	struct files_struct *copy = NULL;
 	int error;
 
 	error = unshare_fd(CLONE_FILES, &copy);