[PATCH] hvc_console: Add some sanity checks

Check if a vterm was registered before accepting it as a console.
Check that a slot hasn't been probed with a tty in hvc_instantiate().
Check that a slot hasn't been free'ed when handing out console device.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>