audit: Do not send uninitialized data for AUDIT_TTY_GET

audit_receive_msg() sends uninitialized data for AUDIT_TTY_GET when
the task was not found.

Send reply only when task was found.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>