Call __fortify_fatal if printf sees %n.

We've ignored %n for a long time, but that's dangerous too because it
makes it unclear whether the corresponding pointer argument should be
supplied or not.

Remove the ambiguity by just rejecting %n outright.

Bug: http://b/31832608
Test: ran tests
Change-Id: Ic046ad3436a30c6f8f580ea738bdcaeb01c858f8