The dev_refthread() in the tty_gettp() may fail, because Giant is taken in the giant_trick routines after the dev_refthread increments the si_threadcount. Remove assert, do not perform dev_relthread() for failed dev_refthread(), and handle failure in the tty_gettp() callers (cdevsw tty methods).
The dev_refthread() in the tty_gettp() may fail, because Giant is taken
in the giant_trick routines after the dev_refthread increments the
si_threadcount. Remove assert, do not perform dev_relthread() for failed
dev_refthread(), and handle failure in the tty_gettp() callers (cdevsw
tty methods).

Before kern_conf.c 1.210 and 1.211, the kernel usually paniced in the
giant_trick routines dereferencing NULL cdevsw, not taking this fault.

Reported by:	Vince Hoffman <jhary unsane co uk>
Debugging help and tested by:	pho
Reviewed by:	jhb
MFC after:	1 week

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