comedi: ni_routing: tools: Check when the file could not be opened

- After fopen check NULL before using the file pointer use

Signed-off-by: Ruffalo Lavoisier <RuffaloLavoisier@gmail.com>
Link: https://lore.kernel.org/r/20240906203025.89588-1-RuffaloLavoisier@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>