IIO: Documentation: generic_buffer example: Avoid NULL pointer dereference

In case optarg n is not given return/exit

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>