staging: dgnc: don't forget to check if ->channels[i] is NULL in dgnc_tty_uninit()

Add a check if ->channels[i] is NULL because a NULL pointer may be dereferenced
in case one of the allocations failed

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>