Minor changes to execute_cable.
Minor changes to execute_cable.

Most prominent changes:
 - checks for missing output before doing any integrity checks
 - assigns output_SD = input_SD earlier when dealing with trivial
   cables that have missing output

cfe-lab/Kive,cfe-lab/Kive,cfe-lab/Kive,cfe-lab/Kive,cfe-lab/Kive