staging: comedi: serial2002: fix Coverity "Explicit null dereference"

serial2002_setup_subdevices() initializes each subdevice based on the
config read from the attached serial device. Part of this initialization
is to setup the subdevice range_table_list for the non digital subdevices.
The range_table_list is allocated only when a 'range' is passed to the
functions. Each channel of the subdevice then has it's 'range' initialized
and that range is added to the range_table_list.

The logic of this function works but causes Coverity complain about an
Explicit null dereference of the allocated 'range_table_list'. Add a check
for the 'range_table_list' to quiet the Coverity issue.

Reported-by: coverity (CID 1011632)
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>