Fix locking race in ttymodem(). The locking of the proctree happens too late

Fix locking race in ttymodem(). The locking of the proctree happens too late
and opens a small race window before tp->t_session->s_leader is accessed. In case
tp->t_session has just been set to NULL elsewhere, we get a panic().

This fix is a bandaid until someone else fixes the whole locking in the tty subsystem.
Definitly more work needs to be done.

MFC after:	1 week
Reviewed by:	mlaier
PR:		kern/103101