afs: Fix check for NULL terminator

Add a missing check for reaching the end of the string while attempting
to split a command.

Fixes: f94f70d3 ("afs: Provide a way to configure address priorities")
Reported-by:  <syzbot+7741f872f3c53385a2e2@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=7741f872f3c53385a2e2


Signed-off-by: Leo Stone <leocstone@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/4119428.1753108152@warthog.procyon.org.uk


Acked-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>